Path of Exile is set up such that there are penalties for fighting monsters lower level than you. If you fight stuff more then 2 levels lower than you then the enemies drop fewer currency items. This is to keep people from just AE farming super low level zones, and I can see how it makes sense. They've put a cap on this penalty as well, so if you're higher than level 68 you're treated as being level 68. In this way a level 66 (or higher) zone will always drop full loot no matter what level you are. Since the level cap is 100 and the highest level zone is 78 you can see how that cap is a good thing. You don't want high level players to be unable to get loot!
There's also an experience penalty for killing monsters lower level than you. The level difference before the penalty kicks in changes based on your current level and it lets you get full experience from any monster within 3+rounddown(level/16) levels of you. I'm level 80, so this means I get full experience from monsters between levels 72 and 88. The experience penalty gets harsher the further you get away from that range. A level 66 monster only gives me 35% of the normal experience. That's a pretty big penalty! All gems equipped to your character earn 10% of the experience you do, but this experience actually isn't hit by the level difference penalty.
What I've been wondering is where I should be fighting if my primary goal is leveling up my raise zombie gem. How does monster experience scale relative to speed of killing the monsters?
Unfortunately that information doesn't seem to exist. Anywhere. I just spent a couple hours searching Google, and Reddit, and the official forums. Neither monster health values nor monster experience values could be found. This makes me sad, because I like when there are numbers I can crunch.
Why don't I just go do some testing and get these numbers myself? Well... I can't. I have NO IDEA how much damage any of my stuff does. I can summon zombies, and skeletons, and specters, and these weird flaming skull things. I can add on gems to multiply the damage they do. But nothing will tell me how much damage they do base, or after multipliers, or anything. I've stopped using the flaming skulls... But I have no basis for that decision. I just wanted to clear up space on my bar and didn't like the looks of them I guess.
I guess what I should do is start tracking the clear times and gem experience gains for different maps to at least get some sort of data going.