I have played through a few times but this is my first time on insane difficulty. I noticed that it seems to be taking considerably longer to level up than compared to core rules difficulty.
Is this right?
What other effects does playing a different difficulty level have?

Actually, if you go to change the setting there should be a dialogue box that tells you what it exactly does. I dont think it takes away your XP and make you level up slower. I think the monsters just do double damage, and a few other stuff.

Ok really. Mmmm, ok question, does anybody remember how much xp they got for killing a demon when you need a demon heart?
On insane I remember getting 16 000 xp but I was sure that was higher playing core rules. Maybe I am just insane but I swear I have less party members this time through and I am still taking longer to level up than before. Well if anyone is up to the before mentioned part in the game please check it out.

Depends which demon. The unnamed ones (called Tanar'ri in the game) are worth 10,000, and the big boss demon, Lea'liyl, is worth 16,000. So yes, you are insane.

I had an experiment about XP in different level. I tested between Core Rules and Hard. The result is the XP is equal on both level. So I believe that it's just only more challenge to play difficult mode.

Ok I'm insane, thanks for clearing that up.