Yes, I think SYN limit contribute to the problem at least.
You can try something like
ab -n 10 -c 1 -k
http://hd-development.net/asfdafd
and
ab -n 10 -c 1
http://hd-development.net/asfdafd
form your pc, one use keepalive, one does not, the keepalive one will be finished in short time, while the non-keepalived one takes forever.
This test uses a non-exist page intentionally, so PHP is not involved. only litespeed internal page.
You can try the same test from that server, both should be finished very fast. If you have a server on the same network, then try the same test from there.
I think it is network issue, not litespeed. if you fix the network issue, the non-keepalive test should take just a little bit longer to finish. If you still have performance problem after the network issue solved, then we check your server from there.
BTW: keep the request rate throttling off before the performance issue has been solved.