Nothing to to but mute certain instruments.
I export my track into multiple layers and then re-import them. It gets rid of VST's which are the main source of CPU troubles. Its also better for mixing I noticed.
I used to make with FL 7 on a pentium 2. It worked...for about 6 patterns. Then I'd have to export it, reimport, add 6 patterns, repeat, load in audacity and fuck with speed, repeat, repeat, repeat, repeat.
Since half a year I have a Dual Core with FL 8 and I still have the habbit of doing this.