Right. Basically, in order for SNEmulDS to run any given part of the game, that portion of the game needs to be in the DS's RAM. The DS only has 4 MB of RAM, so for larger games (3-4 MB), it swaps parts of the game in and out of RAM as needed, sometimes causing slight slowdowns. If you load the entire game to slot-2 RAM, it doesn't have to swap the game all the time.