As far as I know DOTS isn't supposed to increase performance of rendering, at least previously it wasn't a thing, idk when you test it.
If you used it after conversion scenes was added, then you should profile, because I can't belive that performance stayed exactly the same. Was it 20k skinned meshes or 20k simple cubes? Maybe with cubes performance won't change so much unless your try to move them.
That's what DOTS is trying to do. There is no GC and every piece of logic is ideally multithreaded. Garbage collection is unavoidable in vanila Unity, and I'm sure same can happen with Ultra Engine Lua unless you avoid unnecessary allocations and being smart with pooling, but with that GC isn't not a big problem in Unity either.