Short story, don't worry about performance...
Since I never used Fruity Loops, I have no actual facts to go on. But of course, because most of music software magic is number crunching in the processor or sound card, my guess is that you should not notice any performance problems.
The "heavier" software I have used are games. Most performance problems there, assuming Crossover runs them, is mostly because of the lesser performance of the video drivers for Linux (I hear the same goes for some Macs). Most office/productivity software I run basically runs at native speeds, with no "human noticeable" slow downs.
Crossover, like wine, simply translates instructions for Windows to native instructions (Mac or Linux) to allow a Windows software to run. Therefore, Crossover doesn't add much load on the system and is actually negligeable in performance reduction, if there is any.
In short, most of the time, you would need a very precise benchmark test to even detect the difference between running a software in Windows or under Crossover, let alone be bothered by it.
If I should be wrong about this, don't hesitate to open a ticket, the Crossover staff might have a tweak to speed up any bottleneck you might encounter.