GL-MT300N-v2 slow download in STA mode

I have flashed LEDE on my GL-MT300N-v2.

If I connect to an existing wifi network I have extremely low download rates. Upload works fine. Other clients in the same spot have no issues getting any decent speed. The existing network is HT20 enabled. I see upload rates of around 50MBit/s actual tcp bandwidth to a host connected via LAN cable on the existing AP. However download is limited to about a few KBit/s. Tested with iperf3. Other clients achieve normal expected rates on the network.
Another mini router (GL-AR300M) can also easily achieve more than 50MBit/s bi-directional in the same spot.

Downloads from the internet, e.g. opkg update, are dead slow as well on the GL-MT300N-v2.

I ran tcpdump on the AP and on the GL-MT300N-v2. Stream analysis on the AP show lot's of duplicate ACKs and matching retransmissions. There are no lost packets. On the GL-MT300N-v2 I see lot's of lost packets and obviously the same duplicate ACKs. E.g. one packet comes through and then there are a bunch of packets missing in the downstream.

If I run an AP on the GL-MT300N-v2 I can achieve normal transferrates from and to wireless clients with no issues.

Is this some kind of known bug in the wifi driver for STA mode possibly? Anything I could try fixing this through settings?

Thanks a lot!