https://www.facebook.com/Olexi-Photography-142238982536887/photos/?tab=album...44862005607909
There's a load of examples there of shooting indoor in low light with f/2.8 nearly all the time on the 50-135 which is a pain to focus with, very slow. It's still quite serviceable. Not great but not a deal breaker. Certainly wouldn't want to shoot professionally with it though, at least not for this situation. Other than low light sports shooting it's a great lens, but low light action is really demanding on any camera.
Edit: Some are on the 16-50 and 50/1.8 too. A lot are out of focus but in there because they're the best of a bad bunch, it's not the best example but it does highlight the shortfall of this particular camera/lens combination. This is using continuous 9-point pattern AF with medium tracking.