My players are advancing at a rate of about once every one to two sessions. Is this normal, or am I being too nice with more often than not highlighting their major stat?
My character right now advances every, oh, 2-3 sessions, with an occasional 4- or 5-session lull when I'm hanging back and letting other characters take the spotlight. We play 2-hour sessions, though, so if you're playing 4-hour sessions, advancing every 1-2 sessions is about the same.
If you want to slow down - and you might not! - my recommendation is to play out more conversations, not to mess with how you've been highlighting stats.
When I MC, I always make sure that every character has at least one high stat highlighted.