I was playing around with Lightspeed and Gravity today just to see. My base cast speed for Gravity is 2.98s so it gets reduced to 0.48s with Lightspeed up. When I was running around and casting Gravity, I was interrupted about 10-15% of the time. Just something for other to consider if they're used to just running and gunning with a 3s cast time spell. I imagine if you hit a certain cast time it won't happen at all - I just don't know what that cast time actually is.