I don't find that using the official system makes us level up too quickly, often even with combat intensive games we earn like a level every 3-4 sessions. But that may be because we interpret the "first time" xp bonus differently... we don't apply it to kills, only to skill rolls and spell lists (not spells) and it quickly becomes a non-factor after 4-5 levels. Likewise, the players tend to want to avoid getting criticals so they don't lose out on the action, so that reduces our XP too. We play RMSS/FRP btw.