Lubuntu 20.04LTS

The release notes state rather clearly a fresh install should be performed. Yes your Ubuntu base allows upgrade; but issues on upgrade were warned of. See https://lubuntu.me/focal-released/ where it states

"Note, due to the extensive changes required for the shift in desktop environments, the Lubuntu team does not support upgrading from 18.04 or below to any greater release. Doing so will result in a broken system. If you are on 18.04 or below and would like to upgrade, please do a fresh install.

The issues varied on what packages you had installed. Yes it's possible (the box I'm using now is an upgraded box) but whilst some boxes have minor issues, some are not so (it took me 2-3 weeks of fixing issues before I was happy on this box, on some boxes I was happy first boot or with only minor fixes/tweaks)

Lubuntu 18.04 LTS was a LXDE system using GTK2 libs & applications. Modern Lubuntu uses LXQt (ie. Qt5) as GTK2 is deprecated, so different apps are lighter in Qt5 than were used in prior GTK2 releases.

If you're using old apps; the effect on system performance will decline unless you've the resources (esp. RAM) to reduce the impact of having multiple libs that do the same thing co-exist in RAM. I QA-tested 18.04 & later & differences are minimal (esp. if you don't waste resources by choosing carefully; and that included 1GB ram & single-core cpu devices up to Lubuntu 19.04; 2GB & dual-core 19.10+)

If you wanted to continue using LXDE; preparation (ie. making your system a non-Lubuntu, but pure LXDE one) should have been performed prior to upgrade (so LXQt & modern Lubuntu configs were not installed).

If you want to use LXQt & have a resource limited machine you should adjust your use of the system and not use your old apps that made sense on LXDE, but not LXQt.

If that's your issue though; it's unclear, as you've not indicated if you're trying to still use

• Lubuntu (ie. LXQt)
• LXDE or your older desktop environment & no longer wish to use Lubuntu
• your issue relates to the unsupported upgrade; though most issues are minor & petty & those generally didn't impact performance; though some may (why the upgrade was unsupported & the prepared documentation on the upgrade steps was never fully published)

FYI: The last Lubuntu 20.04.3 release notes no longer mention the upgrade from 18.04 as unsupported or to be avoided; as Lubuntu 18.04 LTS had already reached it's end of life, but prior release notes included the warning.