From the FAQ:

For best performance, we recommend to store icons in a ZIP- or 7z-archive (with little to no compression, i.e. zip -0 *), and to store all other images folder-based / as regular files!

A ZIP with no compression is the fastest.

I'll look into the possible bug tomorrow...

A mind is like a parachute. It doesn't work unless it's open. [Frank Zappa]