psp 4.01 M33-2 lag

I recently bought a psp slim in china and got it modded there too. However, the psp lags when playing any game. It lags almost every three seconds. It is currently running on the 4.01 M33- system. Does any of you experience the same problem?
What can be done?

Enter recovery menu (turn off the psp completely, no sleep mode, then turn it on while holding down the R button)

check Cpu speed is set to default or try different configurations