There are a few issues with the speed calculation, i'm not sure though if you would actually go faster with them fixed or not (it assumes you have more horses, but it also mixes up crates and number of goods in bonus and penalty calculations).
Slow horses give you a 30% speed bonus, fast horses 80%. If all troops and crates can be carried on fast horses only any slow ones you might have won't slow you down.
In order to get the speed bonus you need more horses than troops or crates (1 horse can carry 1 soldier and 1 crate). The current speed calculation has a few bugs though and you might end up with strange results when testing it, will be fixed in the next update.