Turn off reflections or emulate a slower CPU/get a better CPU.

Which CPU is the best for ROM?

That's up to YOU to find out. The PC is a massive trial and error platform.

Yeah, Terra Nova is a pretty demanding game if you crank up the resolution and details. I played through it on a K6-III+ 600 and there were many moments in which there was lots of slowdown. I'd say something along the lines of a Pentium II 400 or early Pentium III ought to be enough for most of the game.

What system are you running PCEm on and what CPU/system are you trying to emulate?

I changed the CPU to Pentium 90, Terra Nova runs fast like DOSBox version.