ASUS RT-AC88U Low routing speed

So I got this thing from the trash bin at work, took some effort (TFTP) to flash OpenWRT but it's there and working well now. When I run a speed test, I'm only getting about 4-500 of the 600/600 of my fiber connection. Observing HTOP, it seems to be only pegging 1 of the 2 CPU cores, the second core about 20% only. Is there a setting I can adjust to help with this? I have single core OpenWRT routers that can handle my full fiber bandwidth. I figured this could too.

Edit: upload is a bit slow too. I normally get 620/620 or so on speed tests.

have you tried enabling software flow offload?
also try enabling "packet steering" under "global network options"
there were some reports it should be capable of full gigabit routing see https://git.openwrt.org/?p=openwrt/openwrt.git;a=commitdiff;h=e291e49da3e04bd1cd707958512e1834c18c54cd

Yes, I did software and hardware offloading, it helped but I figured that broadcom dual core 1.4GHz would handle gigabit routing without magic tricks. I also can still only get about 350-400 up.

nope - it doesn't - remember we're talking about 10+ years old platform so it's kind of to be expected considering not even recent days SOCs can't do gigabit NAT without tricks

pppoe? if that's the case it's taxing the CPU extra

If you really want to keep using this device you could also try freshtomato and its bcmnat implementation (tricks category)

I realize it's older. It's fine as is. And no, not pppoe. It is downstream from my regular firewall.

Hardware flow-offloading is not available for this hardware, so that can't do anything. Broadcom's OEM firmware uses a proprietary network acceleration, which is not available for OpenWrt.

But, as maurer mentions, this hardware is on the older end and limited, as well as rather exotic for OpenWrt - and Broadcom is everything but cooperative.