Comment 10 for bug 1847892

Had the affected machine in here (i.e. "where the router is") for other reasons, so I was able to check those conditions too. As expected, it's a power / antenna / etc issue:

Linux 5.3.0-19-generic #20-Ubuntu SMP Fri Oct 18 09:04:39 UTC 2019 x86_64
Fri 08-Nov-19 03:29
sent 459,277,171 bytes received 35 bytes 7,467,922.05 bytes/sec

iwlist scan, WHILE the rsync was running (so it shouldn't be power-saving at all) with the router 6' away with clear LOS showed:
                    Frequency:2.422 GHz (Channel 3)
                    Quality=62/70 Signal level=-48 dBm

Looking at 1795116, I used to get 70/70 at -32 dBm upstairs and through half a dozen walls, so obviously this is a significant drop in link quality given the hugely better conditions.
Compared to 4.15.0-36 - which, bear in mind was one of the BROKEN kernels, the performance is 7,467,922 / 11,167,414, or 66.9% of what it should be in that scenario.

The good news is, that difference suggests it's at least not a simple merge regression of 1795116. The bad news is, of course, that it'll need a new round of investigation track down.

As I've said, I'm willing to set things up to help out, but I'm still waiting for you to provide me with the ACTUAL Ubuntu kernel tree to bisect against. In the meantime, I'll probaby try a couple of the mainline builds since I can just dpkg those, but with no way to map the working Ubuntu ones to the mainline tree I'll never have a baseline to compare against in case something stupid has happened like one of the antenna connectors has become disconnected or etc.