Hey,
when I assign a certain ram usage (8G) and start minecraft with iris & sodium, it changes it back to default usage (2G).
I don't understand why and hope I overlooked something, but maybe it's just a bug.

yes in the iris profile. on installations, then iris&sodium (iris-fabric-loader-0.11.5-1.17.1), more options, jvm-arguments "-Xmx2G-" to "-Xmx8G-"

Never set ram to more than 4GB, especially when only using mods like iris,

why?

More ram actually slows down your PC and hurts the game after a specific point.

doesn't it only slow down the pc because it's using it for mc, therefore mc is "faster" and why does it hurt the game

Java isn't good at utilizing more ram than it needs, thus GC takes longer causing stuttering and overall slowdowns. In any case, what this issue says shouldn't be Iris specific, as Iris shouldn't be able to mess with the profile.

ok thanks

I tried it with optifine and vanilla and this doesn't occure with them, so idk

Known issue. Fixed in next release

Wait i misread this. Are you sure you're setting the memory in the iris profile, not just the defualt Minecraft or Fabric one?