OpenWRT on WRT1900AC v1

Hey guys, im using the WRT1900AC v1 with the latets stable release 22.03.2 and everything seems to work great except for the wan link speed. Its not taking me over 300mbps whereas my ISP link is 500mbps. Using the OEM fw partition i am easily able to hit 475mbps using wired but the moment i switch partition to OpenWRT the speed takes a 25% hit.
My WAN protocol is PPPoe - i have toggled SQM and NAT Offloading but its not really making any difference.
Is there anything you guys can recoemmend to resolve this? I dont want to use the OEM fw as thats rubbish.

start by running htop via ssh, see what the cpu load is.

an update on this - it doesnt seem to be slow pppoe as i installed speed test on the router and they gives me the full 475mbps. So the issue seems to be related to the Lan --> Wan interface...

thanks dude, CPU load is good. doesnt go over 3% while running speed test. Averages at 1.2%.

from where ?
client, or 1900 ?

CPU utilization on the 1900 when running speedtest on a client machine. it doesnt go over 300mbps..
If i run speed test on the 1900 itself using libre then it gives the full 475-480 which tells me that Wan link is good and there is something b/w LAN & Wan causing this bottleneck.