I have now tested 8.1.5 and it is definitely better than 8.1.3+ but still not usable in my environment.
The connection is now at least reliable in most cases (no SSH timeouts, etc.) but still very slow with max. 300kB/s (~3Mbps) over SCP. After ~100MB of transferred data the connection again gets very unreliable and repeatedly drops the SSH session.
I have a 802.11n 5GHz network with an Asus RT-N66U AP with tomato-shibby firmware. Client is a Beelink Mini MX3 II. I also tried switching Channels but it makes no difference. About 1m away from the box my tablet is getting 80-90Mbps.
Edit: With 8.1.2 everything is fine again.