We (FreeBSD) tcp stack can be improved to handle out of order segments more efficiently. I went through some rfcs and based on my limited knowledge, I tried to compile a list of things we can do to improve: https://wiki.freebsd.org/TransportProtocols/tcp_out_of_order
@rrs, can you check how many of these changes were accomplished in base r338102?
I suspect this was probably caught by jtl's changes in 2018, however if the issue is still around please reopen this bug.