It's possible that it's legitimately running out of CPU.
So, I ran a quick test on my machine, and sure enough, Particle Fire is a lot more efficient with multi-coloured fire turned off. On my machine, it takes 6% of the CPU with it enabled, and only 4% with it disabled.
Even this might seem a little high. Back when Particle Fire was written some 16 years ago (!) it was written using Microsoft's GDI library, which was state-of-the-art at the time, and it was hardware accelerated until Microsoft redesigned the graphics stack with Windows Vista. Microsoft kind of abandoned GDI at that point; it still works, but they disabled hardware acceleration, which means it's putting a lot more strain on the CPU than it used to.
But why would it max out at only 9%?
Well, Particle Fire is a single-threaded program. Ten years ago if you had a 1.8GHz processor, a single app would be able to use 100% of the processor, but now your processor has four cores, so a single-threaded app can only use 25% of that.
But you're still nowhere near 25%, right? Well, now we bring in hyper threading. Your CPU has 4 physical cores, but hyper threading turns those into 8 logical cores, so a single-threaded app can only use 12.5% of the total CPU available. Now things are getting dicey; if you were reading 9%, it's not unreasonable to think that sometimes it gets up to 12.5%.
If you go into your BIOS, there might be a setting to turn off hyper threading. This should enable Particle Fire (and other single-threaded apps) to use a full 25% of the processor.