For a single shot, not that much, as the projectile is on it's way before any major movement of the barrel. But coordinated fire with multiple units become pretty much impossible as you need to realign the barrel after each shot. And, obviously, that thing is a hazard to the operators, so it slows operations down quite a bit.

But as shown here by Finnish military video it doesn't have to be like that, you can use artillery effectively in the winter, even with older hardware. You just need to have auxiliary hardware for mounting the thing in place and have trained personnel to use them. For Ukraine, I don't know if they're lacking one or both of them in this and other similar cases.