How much XP did they have before and after their XP went up? Looks to me like you were already 55 hours in at that point (10 XP going to 11).

As I understand it, XP is a flat multiplier applied to that character just as you'd expect. The problem is that nearly everything in CotLI is multiplicative, while XP gain is additive. XP multiplier would be calculated by DPS X (1 + XP) That is to say, 1 XP is 1 + 1, or x2 while 2 xp is 1 + 2 which is x3 of the *original value*. But it is only x1.5 from your previous value because you've already been sitting at x2 for the last 5 hours. 3 XP is only a 33% increase from 2 XP, 4 XP is 25% more than 3 XP and it just keeps getting worse as you go.

Going from 10 XP to 11 is only a 9% increase in dps.

Meanwhile, every 25 levels your crusader gains gives you x4 cumulative damage. Meaning x4, x16, x64, x256, x1028, etc. At lower levels you can get x16 in a couple minutes, while it takes you 45 hours just to get x10 from XP.

Personally, I feel like XP is either not meant to help much beyond new players, or it needs to be worked on.