It keeps getting weirder.
If I keep building the selection at 100% zoom, it slowly builds like in that video over minutes and I'm stuck with a certain amount of memory being used forever like that 9GiB->2GiB in my video.
GC does nothing and so on. Another time it got stuck at 1.1GiB and after 5 or so minutes, started slowly 2-4MB/s dropping down to 910MB and stayed at that for 15 or so minutes.
But if I build the selection at for example 150% zoom, memory consumption goes up way faster than at 100%.
In maybe 2 minutes, I got to 13GiB of my 16GiB of RAM.
And shortly after going over 13GiB, it most likely GC'd to 3.5GiB.
After that I finalized the selection and paint.net dropped to a normal 200MB.