While I'm well aware attack speed affects cast speed (for at least most spells) I'm more curious if it applies a speed penalty BEFORE or AFTER casting.

Till recently I was using a wand and off-hand magic item which gave me a pretty nice 1.4 base attack speed, but when I switched to a staff with 1.0 attack speed, obviously some things go slower.

One of the tactics I've been using involves diamond skin, and basically waiting till the last second to fire it off. I've noticed you can cast it WHILE channeling powers like Disintegrate, so I use it to let me kill off as many baddies as I can before I have to worry about kiting anything.

But since switching to my staff, it looks like there's a little more lag time between tapping my Diamond Skin and when it pops up and protects me.

I've also used Wave of Force in the same manner, to interrupt big enemies mid-swing, and that split-second time difference can be a killer if it's really there, but maybe I'm just imagining things, and I was having a bad day, or had a bit of latency or something.