e1000e performance 30%+ slower on laptop Ubuntu vs Windows/Debian

[nate]

Hello there

Short version of story: using iperf and iperf3(where available) testing 
between
Lenovo P50 laptop and custom built Devuan server(both have been in use 
for years),
performance on Ubuntu and Ubuntu-based Mint significantly slower than 
Debian,
Devuan(Debian-based), or Windows on the same hardware.

"Fast" is 900Mbit/s or better
"Slow" is 600-700Mbit/s

Fast:
-------
Windows 7 [fastest @ 973Mbit]
Debian 10.0 (Live CD) [~930Mbit]
Devuan 2.1 (Live CD) [~930Mbit]
Devuan 3.1 (Live CD) [~930Mbit]

Slow:
--------
Ubuntu 20.04 (Live CD)
Mint 17.x (Installed locally)
Mint 20.x (Installed locally, normal OS I Use)
Mint 20.x (w/Devuan 3.1 kernel+e1000e driver)

I believe I have ruled out:
---------------------------
Kernel version
Driver version
Network interface hardware + firmware
Network cable
Network switch

Which leaves: Some configuration setting(s) in Ubuntu that are messing 
this up.
  (I have done a diff of sysctl settings and tried adjusting a few to no 
avail)

More info:

Test target system is a Devuan 3.1 system running the same e1000e driver 
with
(according to lspci) the same Intel network chip, though while the 
source system
is a laptop, the target system is a Gigabyte Xeon-based home built 
platform.
Regular CAT6A network cables short cable runs(everything in the same 
small room),
switch is an Extreme Networks X460-8t. Everything on the same local 
network
segment.

I've been using Linux for 25 years now, super rare for me to need or 
want
to tweak the default network settings. This issue has been a minor 
problem
for me for years, not critical but today I decided to go down the rabbit 
hole
to see if I could find anything more useful.

As noted above this also affects Mint 17, which is I believe based on
Ubuntu 14, I was using that for the first few years of having this 
laptop,
still have Mint 17 on the boot menu in the rare case I want to try 
something.

Also of course I know that Ubuntu is Debian-based as well, so the 
situation
is quite confusing to me.

I have done a bunch of web searches but nothing useful comes up(I found 
reports
of e1000e being VERY slow as in sub 10Mb that appears completely 
unrelated to
my issue though), have read through the driver README, and as mentioned 
above
even took the kernel and driver from a working Devuan 3.1 system, 
installed it
on Linux Mate 20 and the performance was still "slow".

99.999% of the time this loss in performance doesn't matter to me, but 
something
today got me interested to try to fix it(or at least understand why), so 
hoping
someone else out there has a suggestion as to what the issue could be.

thanks

nate