Apparently the problem was that the zpool was created incorrectly. Moreover, I had to re-create the zpool on almost all of my systems. However, this did not help much, as Alain De Vos predicted when he said that
Before rebuilding, I reinstalled all packages from the official repository with pkg upgrade -f --repository=FreeBSD. For some unknown reason, it didn't start working any faster, and this temporarily averted suspicion from the packages I built on my poudriere binhost.
Later, on this binhost I rebuilt all packages with OPTIMIZED_CFLAGS support where applicable. Finally, the video (and even switching tabs) in Firefox slowed down due to the disabled OPENGL support in ffmpeg. It's worth noting that I didn't even try to plug in Optimus, as this laptop used to work just fine with Gentoo without it.
As a result, everything works fast, and I remove Linux from the 3rd SSD and give it back to l2arc )