I've heard ck850's are very good but I really didn't get along with mine, chipped loads. If I was trying to do a particularly thin L cut (20x10 around a socket) then I had no chance at all, vibration just broke the tile. Maybe I had a bad one.
Try ATS diamond blades. A quarter of the price and better. I have been buying ck850's for years but no more they've gone crap, try the turbo blade from ATS.
turn the grinder on and off quick and check the blade as I slows down it may have a kink in it as mine did the other week, instead of a 2mm cut thickness it was doing about 4mm