@Kevin_Z Hi Kevin_Z, and thanks for your reply. I'll try to summarize:
The AX55 is connected via cable to an Ubuntu Server, which in turn serves the rest of the LAN primarily via WiFi (5GHz and 2.4GHz depending on client).
I've connected to the AX55 using a Win10 desktop on the 5GHz band. When connecting from that same desktop to my Ubuntu server's SMB-server, I get speeds around 75 - 110MB/s (typically consistent speeds around 85MB/s - 90MB/s) When connected to the router's SMB server, the speed drops to 10 - 15MB/s.
I've also tried transfers between the Ubuntu server (which is connected to the router via cable) and the AX55, with essentially the same results.
I've tested different USB3.0 (also USB2.0) sticks and different USB-HDD's connected via an USB3.0 interface, with negligable to no difference. It might also be worth noting that I get speeds around 40MB/s from the Ubuntu server's USB2.0 port when connecting to it via SMB(!), which would be 267% to 400% faster than the speeds the AX55's USB3.0 port manage to deliver.
And yes, filesystems tested - just about every filesystem but ext4, which the router for some reason doesn't support (well, obviously not quite every filesystem): NTFS, FAT32 and exFAT.
In my opinion, this is some sort of firmware issue - if that's where the SMB config is hidden on the AX55.