By left swing you mean right to left? (reverse for right)
It seems kind of goofy... I kind of understand reduced turning in those directions for after release only. Why would you turn slower carrying a weapon normally compared to readying it? And even then, the speed reduction shouldn't be that harsh, maybe enough so you can't do a full 180 overhead before the guy behind you that's right attacking.
And even then, how will this change help make battles more interesting? Overheads/thrusts will have a greater risk for being less accurate. Opponents will also try to move in the opposite direction of swings in order to get out of the sweetspot range. There will be a little more predictability in movement, since there are fewer optimal choices when committing to an offensive action.
I think it could be interesting if the turning reduction is slight.