vst performance meters on the red zone :(

i cannot add any inserts (like reverb) on my tracks without going into the red zone and hearing cracking noises and sound interruptions,
i have a decent computer though (but less than 2 GHZ CPU), a project with few tracks, good RAM memory, have tried high and low buffers,
i still need to check the bit thing (32 or 64, OS is 64),
any ideas ?
thank you