There are several contributing factors which will affect the speed at which your web page will load.
(1) The speed of the web server hosting the site.
(2) The speed of your own internet connection.
(3) The number of external files (JavaScript and CSS) that your web page loads.
(4) The file size of the gallery images.
(5) The value of the imagePreloading configuration option.

You have little or no control over #1 and #2 but you can certainly check the others.

#3 Check that your web page loads only external resource files that are required. If there are any redundant files (for example JavaScript libraries which are not actually used on the page), then remove them and this should help with the initial load speed.

#4 Make sure your image file sizes are not too large. When creating a gallery with JuiceboxBuilder-Pro using the default settings (of 1024px x 768px at 80% quality), the resulting images will likely be approximately 120KB each.
This is usually a good compromise between file size and image quality for web gallery use.
If your images are substantially larger than this, then you might like to try reducing the file size to see if this helps.

#5 Try changing the imagePreloading configuration option (in JuiceboxBuilder-Pro's 'Customize -> Main Image' section) from the default value of 'PAGE' to 'NEXT' so that when the gallery is initially loaded, only the next image in the gallery is downloaded and cached by the browser (rather than all the images on the current thumbnail page).

Hopefully these suggestions will help.

Many thanks for emailing me your complete gallery folder.

The spacing is actually caused by your images not being large enough to fill the gallery's image area (in a large browser window) and there is a gap at the top and bottom of the image in the gallery's image area. (This gap, in conjunction with the top of the thumbnail padding, accounts for the spacing you see.)
If you reduce the height of your browser window (so that the images need to be scaled down to fit within the image area) you should see the gap reduce until it is the same above and below the thumbnails (the only spacing left being the the thumbnail padding iteslf).

When using imageScaleMode="SCALE_DOWN" (the default setting), large images are scaled down to fit within the gallery's image area but small images are not scaled up (as their visual quality would be reduced).

Either increase the size of your images in your gallery or set imageScaleMode="SCALE".
Your landscape images are currently 640 x 479. The default values in JuiceboxBuilder-Pro are 1024 x 768 and this should be more suitable for your web gallery but you will need to feed JuiceboxBuilder-Pro larger source images in order for them to be resized down to 1024 x 758. Small images are not scaled up during the resizing process.

This should hopefully solve your problem.

Incidentally, if your browser window was much taller than it was wide, then you would also see space at the top and bottom of the main images.
Even using imageScaleMode="SCALE", the images would be scaled as large as possible to fit within the gallery's image area (without cropping) but, if the aspect ratio of the gallery is very different to that of the images, space can appear at the top and bottom of the images (or left and right, depending on the shape of the images).
For this scenario, please see this FAQ:
My Juicebox gallery shows too much space above or below the main image, how do I fix this?

Your gallery is too tall for your images. I would recommend setting your gallery's height to be a fixed pixel value so that the gallery's image area has an aspect ratio similar to that of the images themselves.

This problem may be related to server-side caching.
If caching is disabled on your web server, then Juicebox will have to download the images every time it needs them (as it will not be able to request them from a cache) and this may be causing the problem.
If you have an Apache web server and have a .htaccess file with an entry such as the following (to disable caching), then please try removing it to see if it helps.

Header set Cache-Control "max-age=0, private, no-cache, no-store, must-revalidate"

This may solve the problem (or at least hopefully point you in the right direction).

This might be difficult to achieve. There is no easy way to do this via configuration options. You might need to use JavaScript to remove all click and touch handlers from the gallery. However, I have not been successful in my own attempts to do this so, unfortunately, cannot provide any sample code that you can use.

If your images are stored in an array, then the easiest way to reverse their order is to use the PHP function array_reverse before you iterate over them to create your <image> entries, e.g:

$images = array_reverse($images);

If this does not help, then please let me see the code of your 'config.php' file so that I can take a look at at and hopefully help further.

Please see this FAQ:
My Juicebox gallery shows too much space above or below the main image, how do I fix this?

With a responsive site and a percentage height gallery, you do not know what size and shape the gallery will be (it will be dependent on the size and shape of the user's browser window). However, when using imageScaleMode="SCALE_DOWN" (the default value), you can be sure that Juicebox will scale the images down so that they are displayed as large as possible without cropping within the gallery's image area.
Please note that the size of the gallery is not determined by the size or aspect ratio of the gallery images. It is determined by the gallery dimensions and, if you use percentages, then also by the layout of your page (the dimensions of the gallery's parent containers).

Hopefully the suggestions in the FAQ will help.

With regard to padding, Juicebox has several padding options: stagePadding, imagePadding, imageNavPadding and thumbPadding. stagePadding and imagePadding are set to 0 by default.

If you are referring to the code in this forum post, then you can add your gallery's configuration options as attributes to the opening <juiceboxgallery > tag.
For example, you could change the line:

echo '<juiceboxgallery galleryTitle="Juicebox Gallery">';

... to something like:

echo '<juiceboxgallery galleryTitle="Juicebox Gallery thumbsPosition="TOP" autoPlayOnLoad="TRUE" captionPosition="BELOW_IMAGE">';

Alternatively, you can set configuration options in the JavaScript embedding code as noted here.

To hide the thumbnails and Thumbnail Button on the Button Bar, set the following:
showThumbsOnLoad="FALSE" ('Customize -> Thumbnails')
showSmallThumbsOnLoad="FALSE" ('Customize -> Thumbnails')
showThumbsButton="FALSE" ('Customize -> Thumbnails')
showSmallThumbsButton="FALSE" ('Customize -> Thumbnails')

To have the slide show start automatically, set:
autoPlayOnLoad="TRUE" ('Customize -> AutoPlay')

You can change the imageTransitionType and imageTransitionTime in the Main Image section and you can set the displayTime in the AutoPlay section.

If you want the slide show to loop continuously, set:
enableLooping="TRUE" ('Customize -> General')

Depending on the dimensions of your gallery and images, you might also want to fill the gallery's image area by setting:
imageScaleMode="FILL" ('Customize -> Main Image')

In order for the automated slide show to display on mobile devices, you could force the gallery to be displayed in Large Screen Mode on all devices and in all browsers by setting:
screenMode="LARGE"  ('Customize -> General')

I am not familiar with concrete5 but you should be able to embed a Juicebox gallery in one of your web pages using the baseUrl method documented here.

For example, if you uploaded a complete gallery folder named 'my_gallery_folder' to the root directory of your web space, then you could embed it in a web page using the following code:

<!--START JUICEBOX EMBED-->
<script src="/my_gallery_folder/jbcore/juicebox.js"></script>
<script>
  new juicebox({
    baseUrl: '/my_gallery_folder/',
    containerId : 'juicebox-container',
    galleryWidth : '100%',
    galleryHeight : '600',
    backgroundColor: '#222222'
  });
  </script>
<div id="juicebox-container"></div>
<!--END JUICEBOX EMBED-->

The leading slash in the paths denotes your root directory so the code above should work in any web page throughout your site.

The dimensions of the image will be determined by Juicebox depending on the size of the gallery and the imageScaleMode chosen but this really only affects the height and width dimensions of the <img> tags. Ultimately, the images are dynamically resized to fill the specified dimensions by the browser whose scaling algorithms cannot be changed.
All that can really be done is change the resolution and/or quality of the images in the gallery to see which settings provide the best results.

Images in a Juicebox-Lite gallery are displayed using standard <img> tags (generated by the gallery's JavaScript code) and are scaled dynamically by the browser (as Juicebox-Lite uses the default value of imageScaleMode="SCALE_DOWN") so how the images look is somewhat dependent on the browser and its own scaling algorithms.
Out of interest, you might like to try using a different browser (Chrome vs Mobile Safari) to see if this makes a difference.
Perhaps the best solution would be to feed Juicebox-Lite larger images with a higher quality.
JuiceboxBuilder-Lite has default settings of 1024 x 768 at 80% quality. Try increasing these values to something like 4096 x 3072 at 90% quality (at least for testing purposes). Even if this produces images which are actually too large for a real-life gallery, if you get better results, it might hint that you would benefit from using larger resolution/higher quality images in your gallery than you currently do.
If you were using JuiceboxBuilder-Pro, I would recommend using a Multi-Size Image gallery.
Essentially, your gallery would contain up to 3 different sizes of images and Juicebox-Pro would use the most appropriate size, depending on the device being used to view the gallery. The Multi-Size Image logic also takes into account factors such as retina screens such as those on iPads.
Please see here for more details.

wspollack is right. There is no way to stack the thumbnails on top of the main image (overlapping it).
The thumbnails can be positioned only using the thumbsPosition configuration option (TOP, BOTTOM, LEFT or RIGHT relative to the main image).
It would not even be possible via CSS. Juicebox would not be aware of any such modification and would expect the thumbnails to be in one of the designated thumbsPosition locations. Trying to change this manually may cause problems when the gallery is redrawn (for example when the browser window is resized).

When thumbnails are hidden (via the Thumbnail Button on the Button Bar), the main image is resized to take advantage of the extra space that has become available.

The best I can suggest is that you post this idea in the Feature Requests forum thread for potential inclusion in a future version.