Movie.speed()

I am working on a project that requires a variable speed in video playback.  This can be achieved with Movie.speed().  I have two questions regarding it that I was hoping someone would shed a light on:

- What limits the maximum speed?  For my HD video I see no speed difference once I go over speed(2).  I can go from -2 up to 2 but anything outside that range does not do anything.  Is this a CPU thing so smaller videos could go faster?

- What codec is best for a video that should play backwards and forwards at speeds up to maybe 10x.  ( Matroska x264? )

Best,

Sammi