Batman-adv mesh TCP throttled

I'm using two Unifi 6 LRs meshed together with batman-adv and an ER-X as the central router, with topology as follows:

The batman interface and mesh wireless device all have MTU and MTU6 set to 2304. All wireless connections are Wi-Fi 6 at 5Ghz with 80mhz channel width, and all VLANs are untagged unless specifically mentioned.

As per title, all TCP connections from my PC to U6LR2 are throttled significantly to around 90mbps, whilst UDP iperf3 can reach 300mbps.

Wireless is even worse, when doing iperf3 between PC and Pixel connected to U6LR2, TCP averages at 30mbps, whilst UDP remains at 250mbps(albeit with 3% packet loss) on both sides.

I wonder if this is because congestion control was triggered somehow.

Anyway, I'm looking for a way to resolve this and make TCP bandwidth comparable to UDP. All tips are appreciated!