I was leveling my Season character last night and I'm not sure if things legitimately slowed down in the 50s (a relic of the 'vanilla' D3 when 60 was cap?) or if it was just because almost every game I was in after I hit 50 had 1-2 AFK leechers, until I finally said to hell with that and just started playing single player.
It reminded me of the 'hell levels' from EverQuest where things would slow way down. Things felt good once I got past 60 and seemed to be leveling at a decent pace.
actually you can speed it up lol once u hit 45 with good rng you can roll a level 60 item from blacksmith with 15 reduce level requirement. But overall it slows down because it requires much more xp to kill a mob.