I don't understand why someone would move -say- theme elements to MMC when they get used so much?

This is the equivalent to swapping to disk to free up memory when there's free memory to begin with. Doing something when below 10M I can understand, but once you are above some 20 or so you are only degrading performance - basically running a system that has half the space you do.

Memory management is like blackjack. Go as close to max, but don't tip over. A system with 2 GB of RAM that has 50% free has the performance of a system with 1GB. Newer OSs don't have "free" RAM - that's why the more the better. All "free" RAM is in use by OS for read-ahead buffering, executable cache, shadowing, etc. So an OS with 4G outbenches a system with 1G, even of the 4G has 3G "free".

By freeing the space, you are denying the OS the caching it needs, so all data needs to be grabbed from slow storage.

Moving off 50% of files to MMC means that half the stuff you do works 10* times slower.

Why?

*) Based on wiki guesstimates of 150M/s for on-chip and 15M for a good eMMC.