NETGEAR WAC124 - Wild swings in upload and download speeds through router, but not gateway

See below for context:

I've had OpenWRT on this NETGEAR WAC124 router for months and haven't had any issues. I recently got a massive upgrade to 1Gbps up/down fiber. I noticed that some downloads weren't as fast as expected so I connected straight to the fiber gateway for a speed test. You can see the results in the image. Also, the test through the router took significantly longer to reach peak speed.

I also downloaded some Steam games, and doing that thorough the router only peaked at about half as fast as straight through the gateway, and took about 1 minute to get there. Without the router, speed immediately went to max and stayed there.

I would expect some delay through the router, but is it normal to have upload speed completely unaffected? All other client devices were disconnected during these tests, and every speed test I've done in every condition since install had higher upload than download speeds.

Update, further testing is showing extremely inconsistent download and upload speeds through the router, but always with upload ~10-20% higher than download. I have not had a connection before that has shown variance of ~200-300 Mbps in back-to-back runs. Is this normal? Through the gateway, there's about 10Mbps variance between runs