this one might actually be viable (with only affecting units of the same type), if the speed buff dissappears as soon as the units break the formation to engage.
Foot archers all have the same speed anyway, so they cant abuse their instant transition from formation to engaging.
Knights do get kind of a speed bonus by pairing with scouts.
So if you control them to keep them in formation while doing a surround, this bonus is quite a buff.
If you dont, its only a buff to get into 4 tiles range of the enemy, as somewhere around this units break formation to engage.
Regarding infantry celts show us that militia line with the speed of spearmen doesnt matter.
Also to make use of the bonus you need to have a spear in the first place, so drush isnt affected.
If does need a special condition for converted units, else getting a plumed or rattan archer would massively buff a group of crossbows.
And maybe the civ shouldnt get battle elephants.
Overall i like this bonus, as it scales a bit with the skill of the player using it.
Unfortunately its also quite complicated