As I think someone said before, export cpu intensive VSTs to .wav files, then load the .wav into your program.
Use the send effects, so that you use only one reverb for 10 vsts instead of 10.
Another VERY good tip in FL is to go to tools>macros>smart disable for all effects. This stops effects from being used when no audio is being sent through them ( you might be prosessing audio that isnt even there!)
As someone also said before, press F10 and set the buffer length higher under audio ( in FL )
Also, get yourself an asio driver for your soundcard (if you dont have one already) at your soundcards producers website, and if it doesnt have one go to www.asio4all.com .
Hope this helps!