I've tried PPSSPP and the performance does indeed vary per game but is ultimately quite useable. It uses the dynarec dynamic compiler, mips to arm should be much quicker than x86 to arm. It's also proof that the PS2 could definitely be emulated as well using this exact compiler as it used a mips processor as well and was slightly slower than the PSP. I think the main reason PS2 emulators are behind on mobiles are the size of the isos. Games could average 4gb instead of 600mb for the PSP.