Slow 'real world' network performance

[Peter Miller <feurry%gmail.com@localhost>]

When I test my VPS server (virtualized server) with iperf3 or
speedtest-cli, I get 700 Mbit/s. (it varies of course depending on
network congestion) If I run a test server like this

python3.12 -m http.server -b $my_ip

and then download 100MB and 1000MB test files, I only get 80 Mbit/s. I
have seen as bad as 4 Mbit/s, when seconds later I can test 400 Mbit/s
on iperf3.

These aren't one off results. I have 3 virtual servers with different
hosts and locations, and the performance gap is similar on all of
them. With Linux, FreeBSD and OpenBSD I get download speeds inline
with iperf3 test results.

Last night I did my python server test on the same server, downloading
the 1000 MB file to my FreeBSD laptop and got:
NetBSD 160 Mbit/s
FreeBSD 640 Mbit/s
OpenBSD 528 Mbit/s (it got to 640 Mbit/s but averaged 528)
I didn't test Linux (Debian) last night, but it's usually similar to FreeBSD

Does anyone have suggestions on where to look next or some settings
that might help. I have tried the TCP settings suggested here
https://wiki.netbsd.org/tutorials/tuning_netbsd_for_performance/
but this post looks out of date, and it didn't increase performance.

-- 
Thanks
Peter