leveling doest go to a snails pass in an instance lower level skills compared to your average level of skills will not add as much to your leveling but thats as far as it goes

level 45-50 is just where enemys cap in terms of scaling after this point only guards will still scale

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.