The rendering time is the time the last (partial)redraw took to complete. This time (rounded to ms) is then used to give a raw fps, this value can fluctuate greatly depending on activity and performance.
But it's only a raw guidance, I could make it more precise in the next version by using floating point internally (so it doesn't go from 1000fps to 500fps to 333fps etc). Although the fact it's fluctuating this much is the result of it being fast (the slower, the more constant the fps should be). So I'm not sure why you still find it slow. It will also redraw on any change (e.g. different hot part).
But you shouldn't be to fixated on the fps, especially while rotating, placing parts etc is fluently.
But it's only a raw guidance, I could make it more precise in the next version by using floating point internally (so it doesn't go from 1000fps to 500fps to 333fps etc). Although the fact it's fluctuating this much is the result of it being fast (the slower, the more constant the fps should be). So I'm not sure why you still find it slow. It will also redraw on any change (e.g. different hot part).
But you shouldn't be to fixated on the fps, especially while rotating, placing parts etc is fluently.