What i've found is aslong as the rom size + cache size doesn't exceed about 20MEG the game should run fine.
For example SF Alpha 2 is 13MEG + 6MEGS of cache and it runs fine. However if you try Alpha 3 23MEG + 10 MEG of cache size it lags alot. The reason for this is anything over 20MEGS and it has to keep loading from the mem stick. If only they was a way to compress Marvel Vs Capcom and its cache to less that 20MEG instead of 33MEG. Anybody got any ideas?