it appears that if you send frequent time messages and rate is not set to 0, this results in a substantial framerate drop (from 50 to 30 on a G5 quad 2,5GHz in this example).
I'd say that on every bang jit.qt.movie evaluates the current frame something like
frame = previous frame + (current bang time - previous bang time) * rate * timescale
Mac OS X 10.4.5
4 x 2.5 PowerPC G5
1,5 Gb ram