You can change the game settings that govern XP and leveling with the "setgs" command. "iXPBumpBase" is the gamesetting that determines how much XP is added to the cost of each level up - the first level costs 200 XP, and every level after that adds 75 XP to the cost of the last level (level 2 requires 275 more XP, level 3 requires 350 more XP, and so on). So, if you wanted to level up half as fast, try:
"setgs iXPBumpBase 150"
That'll double the cost added for each levelup. Try messing with the number to change the results - the default is 75. Also, I'm very not sure how this will affect a character that already has a few levels under their belt. I'm also not sure this console command will even work... but it should. Just, save before you change this value and make sure it works how you want.