Lol illum, this is a really simple build. Sorry, I'm not very good at explaining things (migas knows, back when I tried to explain rock paper scissors... XD), but this is easy enough for someone that just started to play Minecraft to build. Just a line of hoppers is all you need, then a comparator that connects to a light. It wouldn't lag anything at all.
You're right though, they would need XP... trying to think of a way around that situation.
You could always have some shops that sell 85 XP for a few bucks. Because if you are at 0 XP, you only need 85 to get to level 5.
It was just an idea though, I thought it could be cool to see a semi-automated trivia. But don't worry about lag, though. XD