In game lag

[el_rika]

Due to overheating, your CPU might be clocking down (it is a built in preventive measure). You should install a CPU tweaking and monitoring software to see if there is indeed a connection between the CPU temperature and the lag.
Rooting the phone certainly helps (with the right app you can also manually set a higher limit for the temperature at which the CPU throttles).

There is also a memory issue with PPSSPP in android which causes random lag, in my experience. Depending on the game you might need to tweak some settings in the emu's menu, but for starters go for 1 x resolution and leave any enhancers disabled (effects, filtering).