EDIT: I was running cake whist capturing speed tests, otherwise download was 900+ mbps
NB: Part of why I don't run v21.0x as it only uses a single CPU in my WRT1900ACS which I feel degrades throughput
...whereas this has been an issue for some of us since at least mid 2021 on Marvell silicon.
I have rolled the latest master and am still restricted to the lowest speed NIC on the LAN at 100mbps on egress. When running a speed test from the router itself there is no issue - something restricting the LAN (and causing the whole device to lag as navigating LuCI becomes nearly impossible)
I have a similar issue on TP-Link Archer C6 V2 with OpenWrt 21.02.3.
The speed of up-linking, PC-(2.4 GHz WiFi)-> Archer C6 -(5 GHz WiFi)-> Buffalo router -> Internet, has been limited < 10 Mbps at any time.