It seemed like I had to kill 30 enemies or nearly two dungeons worth of content just to gain a single point of advancement in either destruction or conjuration and thats when those skills were still in the 20's. This has a negative side effect on your overall power as a mage because when these skills are building slow you often end up leveling many of your other skills (though not intentionally) such as lockpicking and sneak several times before you're able to raise your magic skills. This even with the Mage stone enhancement buff.

On my warrior, the only skill I noticed that builds semi slow is heavy armor, other than that I frequently get 2 or 3 skill raises of 2h weapons and block per dungeon I go into, even smaller ones. So its easier to control your leveling as a fighter type which in turn makes you more powerful as you level. I don't know why they decided to make magic skills build so slow but it seems out of balance to me, just my opinion,