There are a few things I know of to help reduce the CPU...
1) Increase your buffers. (Options>Audio Settings)
2) Lower your playback mixing rate. (options > Audio Settings). Change Interpolation to Linear.
3) Tunr on smart disable. Not in FL6. Click on the top left corner of the mixer and select the option.
4) I have noticed some VSTs loaded multiple times (many that were created with synthedit) seem to chew up a lot of CPU once the second instance kicks in.
To find out if it's a specific effect, just go to each channel and disable the effect until you find it. I noticed that I had one guitar amp simulator that always caused a huge spike in CPU.