I have a low-spec laptop (formerly with Windows 10) with an Atom-derived Pentium N4200 in an Acer Swift 1 laptop. Even with its low level of performance, it has run KDE Plasma nicely for the time I have had it (about 2 years), since Plasma is lean and mean these days.

After KDE Neon updated to Plasma 5.20 (currently 5.20.1), bringing the new task manager plasmoid that has the "cycle through windows" mode as default, I unselected that in the task manager settings and selected the textual list option for what to do when a stacked taskbar item is clicked. This is in the regular style task manager, not the icon-only style (I am bewildered by the icon-only mode)..

If I have a number of, say, Dolphin windows open (enough to stack), perhaps six to eight, when I click the stacked item button on the taskbar, it takes about two full seconds for anything to happen. When I select one of the items in the popup menu that eventually appears and click it, it takes a few seconds for that item to open too. It's never been anything close to that slow before the update.

I copied over the plasmoid (the whole folder) for the task manager (org.kde.plasma.taskmanager) from /usr/share/plasma/plasmoids from an installation of a distro with Plasma 5.18.5 (Fedora 32, as it was what I had handy) into ~/.local/share/plasma/plasmoids for Neon, and after a log out and in, the expected level of performance when opening the list of stacked Dolphin instances was restored. It takes, perhaps (very rough estimate) 200 milliseconds, an order of magnitude faster than with the new version of the plasmoid.

I tried logging in to a brand new user account with all of the defaults, and I noticed the same issue (it used the system plasmoid, since the one from 5.18.5 was only installed into my user plasmoids folder).

On my faster machines, one an i5 (Sandy) and one an i7 (Coffee), I don't notice any delay with the task manager, and I do have it set the same way as in the N4200 machine, including the same version of Neon and Plasma.

STEPS TO REPRODUCE
1. Set the task manager to the non icons-only alternative on a low-spec PC running Plasma 5.20.1
2. Set the task manager to "shows textual menu" in the "clicking a grouped task" field of the settings
3. Open at least six instances of some program, like Dolphin
4. Click the grouped task button 

OBSERVED RESULT

The text menu opens up quickly with very little perceptible delay


EXPECTED RESULT

The text menu takes about 2 seconds to open

SOFTWARE/OS VERSIONS

Linux/KDE Plasma: 5.20.1, KDE Neon User Edition, 20.04 base
(available in About System)
KDE Plasma Version: 5.20.1
KDE Frameworks Version: 5.75
Qt Version: 5.15