Poor performance with PS2 on RP5 (nethersx2)

From my own experience and watching videos online, it seems that PS2 emulation isn't that great.

Sure God of War runs fantastic, but there are so many games (call of duty, NHL09, test drive off road) etc that have horrible frame rate dips, even at 1x resolution.

Is this due to the emulator being shitty or is the RP5 underpowered?

I can run Wii U games at 60fps, Wii COD games look amazing. Gamecube games run fantastic, i just don't understand why PS2 is such a challenge for this device.

Can anyone let me know what I might be doing wrong or if we just need to wait for Nethers to be improved in the future.

Edit: I've changed back to the classic nethersx2 and have experimented some games with open gl and it does make a pretty big difference.

Appreciate the answers to my question & the tips that have helped a lot so far.