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Debian testing linux-image update leads to slow file transfers?

On 03/28 I updated 3 Debian testing systems on our LAN at home,
the update including --


Since that time two behavioral differences have been conspicuous
on all three systems.

The first is that these messages appear in dmesg during every
boot process:

[    1.799465] platform regulatory.0: firmware: failed to load
regulatory.db (-2)
[    1.799498] firmware_class: See
https://wiki.debian.org/Firmware for information about missing
[    1.799537] platform regulatory.0: Direct firmware load for
regulatory.db failed with error -2
[    1.799540] cfg80211: failed to load regulatory.db

The second is that large file transfers on the LAN are almost
unbelievably slow, requiring (for instance) more than three
hours to copy a 5 gigabyte file from one system to another. Such
operations required only a few minutes before the upgrade of 03/28.

I found some information in threads at bbs.archlinux.org that
indicated that this issue was supposed to be harmless, but I
learned, also, that some systems have shown evidence of very
slow file transfers following this kernel update. This
apparently had something to do with a change in TCP congestion
control to bbr.

I ran the following command on all three systems:

# sysctl net.ipv4.tcp_congestion_control=cubic

File transfer speeds returned immediately to normal for all
three systems. Naturally, this did nothing for the dmesg
messages, but system backups and other file transfers no longer
require ridiculous amounts of time.

I saw nothing in the Debian bug tracker that seems to apply, but
I may have been looking in the wrong places. I did fire up
reportbug and tried to send the information to
reportbug.debian.org, but got an SMMTP send failure.

Would like to get the information in front of someone who might
be interested and able to do something about it. Suggestions?