It could be due to you not getting as much experience from mobs. I think at lower level areas, the level of mobs tend to be higher for their actual difficulty, thus making experience gain faster. I'm not sure which area you are adventuring in right now, but it could be that areas you tend to be in when you first hit level 20 (e.g. Kourna) might be inefficient experiment-wise. Don't take my word for it though, since it's just a guess.
If you want to focus level up your heroes to 20, I recommend doing quests since they give you set amount of exp no matter your level. You could also bring the hero along on a mission or primary quest if your party composition and mission requirements allows for it.