When rendering animations with motion blur, all is well for the first 90% of each frame. This is when UF is calculating the fractal and antialiasing. The four CPU cores are maxed out and the CPU is 0% idle, as shown below on Activity Monitor.
But then comes the final 10% of the frame, which is when motion blur is being calculated. UF suddenly gets a lot slower. The CPU is 70% idle. The two physical cores are about 50% idle and the other two cores are are not being used.
I’m using motion blur because it looks great in animations but it’s very slow. Renders which take hours without blur require days with blur. It’d be really great if motion blur used hyperthreading and ran at 100% speed instead of only 30%.