I think it's a bit of a catch-22: given one of Putin's stated aims is to demilitarise Ukraine and remove Western weapons, NATO arming Ukraine earlier would have caused Russia to invade earlier.
Invasions take a lot of preparation. You can't just tell your army to attack some country - they need to plan exactly how they will do it, and practice.
The turnaround time on this sort of thing does not favor the aggressor.
