Simple change: you move 15% slower while holding LMB (windup) or attacking.
More complex: amount of slow depends on weapon you're using. For example 10% for onehanders, 15% for twohanders, 25% for bows.
Why? Seems like having too much mobility during attack makes fights look like jousting on foot (both opponets run to each other while holding LMB and try to hit and dodge) and therefore fighting feels clunky and floaty sometimes. With such a change, a fight would look like a fight and not like running in circles, hopefully . Also it would fix archers who can run around while shooting (if this is still an issue).
By the way, such mechanics is used in chivalry:mw and working out well.