The difference is anywhere between 2-7% faster, 1 second to switch back and forth + 200ms for aiming leeway. Crossbow doesnt have startup time, you can shoot on the first frame & switch back immediately.
There is very little benefit to go for 1 frame arrows imo, not just for crit heal arrows but arrows in general. If you shoot 10 arrows and switch to the medigun on 1 frame after shooting compared to 200ms of leeway you're looking at a gain of less than 2 seconds, so even less than 5% uber.