Detailed info:

I have two computers connected to the same router through a cable,
each one runs two systems.
Desktop:
Arch Linux + Plasma 5.16.3 + Network Manager
Void Linux + Plasma 5.16.3 + Network Manager

Laptop:
Arch Linux + Gnome 3.32.2 + Network Manager
Neon unstable edition + Network Manager

Both Arch and Void installed on desktop computer have
exactly the same problem: internet speed
drops to one third after suspend/resume cycle. Rebooting
is the only way to get the correct internet speed again.

Neon unstable edition always give me one third of the expected
internet speed when I use the ethernet adapter.
Wireless adapter always gives me the correct internet speed,
even after suspend/resume cycle.
Arch + Gnome installed on the same laptop always gives me
the correct internet speed even after resume/suspend cycle
regardless of whether I use ethernet or wireless adapter.

I solved the problem with slower internet speed after suspend/resume
on Arch + Plasma running on my desktop by replacing Network Manager with wicd.

Therefore, all systems running Plasma + Network Manager have issues
with the speed of wired network.
That's why I think that maybe there is some problem between Plasma and Network Manager.

To me it looks like a problem somewhere in NetworkManager, plasma-nm can't really affect connection speed, unless there is wrong configuration. Maybe check your wired connection whether it has "Allow auto-negotiation" option checked, if it's unchecked, then "speed" and "duplex" can be set and it might be wrongly set in your case. I would also recommend trying to re-save your wired connection even if 'auto-negotiation" is checked, as the connection still may be wrongly configured. Just try to change its name or anything and re-save it, disconnect and connect again.

"Allow auto-negotiation" was disabled and "Duplex" was configured as "Full" on all my systems. Is slower wired connection after resume/suspend expected when "Duplex" is "Full"? Or is there a bug with "Duplex" setting?
I solved the problem after suspend/resume on all my systems by enabling
"Allow auto-negotiation" option and disconnecting/reconnecting. \o/

(In reply to Patrick Silva from comment #3)
> "Allow auto-negotiation" was disabled and "Duplex" was configured as "Full"
> on all my systems. Is slower wired connection after resume/suspend expected
> when "Duplex" is "Full"? Or is there a bug with "Duplex" setting?
> I solved the problem after suspend/resume on all my systems by enabling
> "Allow auto-negotiation" option and disconnecting/reconnecting. \o/

I think it might be some misconfiguration, there used to be different default values for "duplex" and I remember people were reporting similar issues. Either re-saving problematic connection or create a new one was solution for this.

I disabled autonegotiation, configured "Duplex" to "Full", renamed the connection, disconnected and reconnected... Tested again and I got the same slower speed after suspend/resume. "ethtool enp4s0" command confirmed that "Duplex" was "Full".
So I removed the wired connection, a new connection was created automatically with Dulplex = Half. I changed it to "Full", disconnected/reconnected, another suspend/resume cycle and I got the same result again.
What do you think Jan? Is there a bug here?

I'm not sure it's a bug or expected behavior, you would need to ask NM developers. Default value for "auto-negotiaion" is false, I don't know why, but it's weird that after suspend and resume your connection is slower, even with "duplex" being set to "full".

Thank you Jan.

A few days ago I replaced my modem and stopped using a wireless router
and now I can't reproduce this problem on any of my systems previously affected.
Interesting...

Bulk transfer as requested in T17796