One of my friends had a problem with much lower throughput of his main router RB450G (doing NAT) after upgrade to 5.8. He had to return back to 5.7. I've barely believed him, but today I needed to do some transfers between my computer and my NAS and the speed was much lower than it was few weeks before as well. I have RB750G.
Situation: my PC <-> RB750G <-> NAS, router is doing routing only (no NAT). I've disabled all firewall rules. If I turn off conntrack, speed is about 100 Mbps, that's good (maybe speed limit of my NAS). But if I turn conntrack on, speed is decreased to cca 40 Mbps and CPU usage is very low most of the time during data transfers. That is much better in 5.7 - speed with conntrack is about 80 Mbps, but the CPU usage is higher - I've tested downgrade today. Did somebody experience similar problem since 5.8?