Basically, further into the round you are, the slower your horse is?
Normal speed, up until the last ~20% of a round, then it progressively gets slower until donkey speed™ is achieved at ~10% of the round left.
e.g. 6 min round; 360 seconds, so for the last 72 seconds (1 minute and 12 seconds) your horse would get slower until hitting donkey speed™ at 36 seconds left.
Just an idea, haven't thought it through much yet, but it's possible to code and could help with the MR (mounted range) problem.
Thoughts?