May fail with ZeroDevisionError

[just-doit]

I had a ZeroDevisionError in line 1101:
speed = float(chunk.block_size) / float(delta)
Since delta is based on time.time(), it can be 0.0 if the system is fast and the file is small or the resolution is too bad.
Under Windows 10 (for me) time.time() returns only two fractional digits.