I don't think that this is a bug, but it is really annoying.

I don't know how exactly Mojang do benchmarks, but 40% time of CPU is spent on a garbage collector.
For instance, version 1.12.2 spends only 1% for GC.
Minecraft's allocation rate is around 100 megabytes per second.

Most allocations are Vec3 objects for math, (why are they not mutable?).

UPD: Yes, 1.12.2 is kinda old,but it's the last version before render engine/codebase rewrite.

UPD2: Allocation rate above is actually not vanilla, it was OptiFine + tons of stuff removed from the client, math objects made mutable, axis boxes made mutable and so on.
Vanilla: