MT7628 wi-fi abnormally slow

I have a TL-WR841N (v13) router that runs the latest OpenWRT version 24.10.1. Its wi-fi clients, connected at link speed 72/72 Mbps (receive/transmit, b for bit) but can only download at max 2.6 MBps (B for byte). However if I attach a 802.11n AP (access point) to the router via Ethernet, and the same client connects to the AP at the same speed of 72/72 Mbps, the download is much faster, with max over 5 MBps.

Here is the details of the two tests:

(1) A laptop connects to the router via 802.11n wi-fi. The router claims to be 300 Mbps, but I guess it's because it has two antennae, and can double the channel width from 20 MHz to 40 MHz, thus quadruples the bandwidth - that is, 72 Mbps x 4 roughly equals to 300 Mbps. However in this test the router channel width is set to 20 MHz only, and I guess my laptop has only 1 antenna, hence the link speed is 72 Mbps (I checked the link speed reported by the OS which is Windows 11, and it reports 72/72 Mbps). At the same time another computer is connected to the router via an Ethernet wire (100 Mbps). The laptop runs an SFTP tool to download a large file from the other computer - the file data goes from the other computer via the Ethernet to the router, and then from the router to the laptop via the wi-fi link. There are no other computers using the router during the test. I'd expect that the download speed to be about 9 MBps (a byte has 8 bits so 72 Mbps is equivalent to 9 MBps). I understand there are some overhead cost, hence it would be somewhat lower than 9 MBps. However the test result is 2.0 Mbps ~ 2.6 MBps during the whole download.

(2) I connect a 802.11n AP device to the router via Ethernet, and then have the laptop connect to the AP (which has a channel width of 20 MHz). Again I checked the link status and confirmed that it's 72/72 Mbps. I then do a large file download. The file data goes from the other computer to the router (via Ethernet connection), then from the router to the AP device (via Ethernet connection), and finally from the AP device to the laptop (via wi-fi). However this time the speed is 4.0 MBps ~ 5.2 MBps during the whole download.

So, it seems that the 802.11n wi-fi inside the router (TL-WR841N) is abnormally slow. It might not related to this particular version of OpenWRT because I don't remember it was any faster before when using version 23.05.x. But what might be the reason for this slowness?

No source file, test, or entry point is named. Start by reproducing the two 802.11n throughput comparisons for the MT7628/TL-WR841N setup and inspect the mt76 issue context; done means identifying a driver or configuration cause and documenting a reproducible explanation or next diagnostic.