Dir-860l Rev B1 very poor wifi performance

I can second the poor quality of the internal antennas of the DIR-860L.

I'd been using a rev. B1 with CC 15.05.1 for about 6 months, when it started to get flakey on me.

Lots of disconnects. Wifi speeds on 2.4GHz dropped under 1 megabit/sec, and on 5GHz under 10 megabit/sec. Before that it was 25-30 Mbps and 35-45 Mbps respectively. Speed through ethernet was fine.

Flashed the unit back to stock and reinstalled CC from scratch. Didn't help.
Flashed it back to stock and installed LEDE 17.01.4. Didn't help.

Then I found this thread and opened the unit. It has 2 small internal "smart" antennas. The one on top had a half severed cable, probably nicked during production. So today I decided to replace both with 2 external 5dBi antennas from Ali Express:


And lo and behold: The wifi speed is back to normal. So thanks for the tip!