Increasing speedup does not actually speed up replay at high values

[BH4]

Would be nice to be able to determine at what speed up value there is no longer any appreciable change in execution time. Additionally it might be useful to check exactly how accurate the speedup value is, i.e. does increasing it by a factor of two actually change execution by a factor of two?

[BH4]

This is directly related to the code execution time not being accounted for and so is made a sub-issue of #8