Clumps

Clumps is great on 1.12 and some older versions, but it's just outdated and unnecessary on newer versions. As a past review said, the way the mod "combines" the XP orbs is in a clunky way, and while there is a slight performance boost on a big modded server or huge modpack or a huge world full of XP farms, many singleplayer users will never make enough XP orbs to effect performance greatly, or for this mod to have a noticeable increase in performance.

Sodium or any other modern alternative and any culling mod will make a far bigger performance boost than XP clumping ever will.

Not sure if anyone realizes, but modern vanilla Minecraft clumps XP orbs too! And it doesn't take a clunky mod!

I have no idea why this mod is one of the most downloaded of all time, sure it's fine but not necessary like some players may think.

This review only applies to the 1.12 version as I haven't tried it on newer versions.
Clumps can significantly improve performance in situations where an area is generating lots of XP orbs, such as in mob farms. This is especially useful on servers. But this mod does it by replacing the XP entity, which can break mods that also rely on them. For this reason, if playing 1.12, you should use Fixeroo instead, which does the same thing without adding a new XP entity.