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Posted the question as quoted below sometime ago, now resolved. Resolution detailed as a reply :

In focal fossa, Top shows 95+% of CPU used by systemd+, which also uses about 20% of memory.

There was a Unix StackExchange thread on this which outlined a solution which did not work.

https://unix.stackexchange.com/questions/494843/how-to-limit-a-systemd-service-to-play-nice-with-the-cpu

Tried

sudo systemctl edit systemd

and

sudo systemctl edit systemd+

Ubuntu 20.04 said

No files found for systemd.service
Run 'systemctl edit --firce --full systemd.service' to create a new unit. 

That command opened a new file, I copied and pasted the settings from

[Service]
CPUWeight=20
CPUQuota=70%
IOWeight=20
MemorySwapMax=0

then

systemctl daemon-reload

Top still shows 95% cpu for systems+

20.04 boots with alternate display resolutions, invariably with some minor boot delay, with one or the errors contributing to the excessive activity by systemctl, the two major recurrent errors being Starting Stopping Tell Plymouth and stopping and starting nvidia persistence daemon. For more context

https://ubuntuforums.org/showthread.php?t=2441923&p=13954300#post13954300

What should I do, please?

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Resolved. I installed a new hard disk, began by installing 16.04, first with an attempt to experiment and understand hard disk encryption, with encryption 16.04 installed fine, but update to 18.04 showed lvm errors, did not proceed smoothly, deleted the crypt partition, re-formatted the entire hard disk, proceeded to reinstall 16.04 without encryption, update to 18.04 was smooth, and as a solution to the display issue, followed a post and installed xfce4 which resolved almost all of the hardware issues that I posted in this and other threads, same old hardware, same samsung 2030 monitor, same nvidia graphics. Then proceeded to update to 19.10, then to 20.4 all by update-manager. With xfce4 there was no need to attempt any fix, such as install an old nvidia driver, or to blacklist noveau, or even a nomodeset, there were no plymouth errors, Top, with no application running showed less than 5% CPU usage, display works at 1600x900 on its own, no boot issues. Then I purged unity (might be unnecessary, but ubuntu's login screen was pixelated before I struggled a bit with the invisible cursor to type my password for a clean xfce environment), installed xubuntu desktop, purged mate, (mate loaded with a corrupt display).

20.04 with XFCE4, XUBUNTU desktop works splendidly with the same 10 year old hardware with limited resources, without any fixes.

Top screenshot of 20.04 xubuntu desktop with 10 year old hardware

Saying this,

(1) in a manner of withdrawing the few posts, that might have given a few readers the impression that 20.04 has some issues. The issues on the older disk probably had to do with the sequence that I followed in upgrading from the unsupported 10.04 installation, by a complex process of installing drivers, enabling third party repositories, and trying any fix found in the forum such as grub edit, blacklist...

  1. This might be helpful to some who reported display issues, plymouth issues on 20.04, the solution could be found in choosing a desktop such as xubuntu or LXDE.

samsung 2030 in 20.04 nvidia graphics, no settings, no driver fixes

  1. In trying to find bug fixes for plymouth or or for some versions of nvidia, the developers may find it useful to focus on the reasons why some display manager work and others don't.

The decision to move away from multiple O/S installations in the same hard disk was based on the advice of a friend who said "it is not good to have more than one O/S in a hard disk)

Some finer problems remain, such as a bug with Thunar, removed thunar, reinstalled nautilus a moment ago, will see if it fixes the issue. Another minor issue is that after long inactivity, the wake up log-in screen gets into a login loop.)

Thank you.

(Edited the question and posted this solution as an answer as suggested by Pilot6. Thank you.)

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