Started here. If my theory is correct, the bug is not tied to --mark; it's tied to the amount of rows pool4 contains.

Even if --mark is at fault, this bug should be addressed because, even though RFC7422 and --mark are (to most purposes) roughly the same feature, and the former should naturally scale more elegantly as more clients need to be serviced, the latter is more versatile since it matches clients arbitrarily by means of iptables rules. So I don't see any reasons to drop --mark once RFC7422 is implemented.

I believe this is the symptom that needs to be addressed:

When there are many pool4 entries, translation rate drops noticeably.