I didn't notice much of a slowdown until about level 71. Probably because I decided to drop my heavy armor and one hander for light armor and two hander to level those to 100 for giggles.
Now (level 75.5) everything is on 100 except speech (96), most magical skills (between 50-92) and alchemy (43) and its only hard to level further because I can't be bothered to level the magical talents with no investment in magicka (i.e. can barely cast a few spells) and alchemy is pretty impossible to level when you know the properties of just about every ingredient already and you need to make 100 potions that you know all properties of to get 1 level in the skill.
IMO alchemy in Skyrim is broken. Just too hard to level compared to other skills and you really need to (remember to) pay a trainer on every level to max that.
Make yourself -100% spell cost gear. It makes it quick to level up Conjuration, Alternation and Illusion. Destruction and Restoration are much faster than normal too, but slower the the others.