Why? If your computer is fast enough to run MAME/MESS/UME it's more than fast enough for QMC2. And especially the MESS variant is not "slow" in any way. So I'm a bit surprised about your statement.
Anyway, SVN r4792 adds slot-info caching which loads about 4 times faster (here)
. So instead of ~600 ms it's now ~150 ms during the initial slot-info loading. Whenever the cache has to be (re-)created, it takes the same time as before, of course.
All rules apply as with the other caches, that is, whenever you change the underlying emulator binary and it's not indicating a new version (i.e. from SVN), you have to remove the cache and reload the game/machine list (it's also included in "remove all caches", or -cc on the command line).