The bearing and piston clearances on a cold engine are way looser than on one that's warmed up, particularly pistons. A cam ground piston only contacts the cylinder wall with a very narrow strip at the center of the piston skirt when the engine is cold, and the egg-shaped piston skirt gets round and much closer to the cylinder wall as the engine reaches operating temperature. That increases drag considerably, and puts more stress on the starter. Most other clearances like all the bearings, the valve guides, lifters, etc. also run with reduced clearances once everything gets hot. That's why we build brand new race engines with more clearance to everything than a 100,000 mile street engine. We don't care how long it lasts or if it burns a little oil, as long as it finishes the race. We just don't want it to tighten up and rob any horsepower as it gets hot. A geriatric race engine that's ready for a freshen-up with new rings and bearings might have 1,000 miles on it, but it spins VERY freely!
BTW, that's also why people who do race engine stuff on a street machine are stuck on stupid!
Jerry
