If, Emperor willing, the dice come up right, the Warp can be reasonably fast.
But...those dice don't often come up right. Ships can end up centuries overdue, if they arrive at all.
Erm...Imperial Guard regiments aren't exactly small formations of soldiers...and there's a thousand other fronts that need constant supplies of manpower.
In the case of Star Wars, it really does depend. A lot of the battles are being fought on sparsely populated worlds of the Outer and Mid Rim, so behemoth 40k sized armies aren't quite necessary.