Is this a real trouble for gallery loading on mobile devices ?
If you want to have a Juicebox gallery on your web page, then there is no option but to include the 'juicebox.js' file (which internally loads the 'theme.css' file). These files are too large to be included inline within a web page and this is not possible anyway as the structure of the 'jbcore' folder must be kept intact for Juicebox to know where its own resource files are located.
Loading the 'juicebox.js' file asynchronously may not work (the 'juicebox.js' file will have to have been fully loaded by the time the browser parses the embedding code <script> block) but if you would like to try it, please take a look at the suggestions in this forum thread.
(Incidentally, entering 'google.com' into Google's own own PageSpeed Insights results in a Mobile Speed result of 64/100.)