'Very slowly and then catching up' - have you tried increasing the number of threads? That sounds like the sort of behaviour I would expect from a small number of threads. To explain this 'threads' thing: Windows splits the processor's time between all the programs running on the computer. Each program (or 'process') can split itself into several 'threads', each of which gets allocated time by Windows. The effect is that several things in your program (in this case UF) appear to be happening simultaneously. The maximum-threads option determines how many separate threads at a time UF can have running for a fractal window; so if you have a small limit, you'll see that only a few layers in a many-layered image appear to be progressing at a time. I don't know exactly how UF splits all its tasks into threads, so I can't be sure that increasing them will fix the gradient window problem, but it's worth a try.