I’m seeing the same problem on 2 Raspberry Pi 2s. The configuration on both is nearly identical because one was imaged from the other, then modified (hostname server ssh keys, etc).
I currently have a 512/512 memory split with the default theme on them. Everything works great as long as I don’t try to have more than 23 emulators active. As soon as I go above that, I get the white screen freeze described above. I can add an emulator as long as I remove one (by swapping out it’s rom folder). It doesn’t seem to matter how many roms any of them have in them, from one to a thousand.
It’s interesting that htop run in another console shows that the cpu cores are mostly idle when the white screen is up.