Well, I've done some testing, and found some interesting points:
1) Seems obvious but can be easily overlooked: size of images impacts on performance, box images downloaded from Emumovies can be very high resolution, so loading and creation of thumbnails can be much slower than using smaller images like i.e. 3D boxes. I plan to mass resize my box images once finished downloading them all.
2) Once cache images are created on disk, performance is much better. Is there a limit on maximum number of cache folder content? Allowing to create a cache of all the collection thumbnails couldbe an option.
(BTW the performance is now much better, have you optimized the caching on latest betas?)
One solution to slow performance when running from a NAS, related to what @Gazoo also suggests, is allowing to have cache folder on local disk, for instance Windows temp folder. This way:
a) Cache images are created faster as Launchbox is reading from the NAS but writting locally (mixed read/write to same drive hammers performance, specially on networked drives, could also allow other users to store the collection on a slower HDD and caching on the OS SDD).
b) Cache images are logically loaded much faster, allowing snappy browsing.
So my suggestion is to add an option to locate cache folder on a local system path, or an option to define this path manually. I haven't found a way to do so, and as it's located on a subfolder inside Images folder, if pointing images folder to the NAS there's no way to point cache to a local path.