Good advice from RedRox. It always takes longer to get around the desert southwest than you expect, just due to stopping and admiring the views...unless you can somehow drive with blinders on.
Is your plan to drive from Bryce Canyon to the Grand Canyon with a stop in Page for the slot canyon tour, all in a day? That's a bit much, IMO. I would definitely add an overnight stop in Page if you can fit it into the schedule. There is a lot to see and do around Page in addition to Antelope Canyon: Lake Powell, Glen Canyon Dam, Horseshoe Bend, Rainbow Bridge, float trip down the Colorado, etc).