SUMMARY
The "get new stuff" section inside global themes, plasma style and colors is extremely slow at loading images. This makes theme selection and download very frustrating.

STEPS TO REPRODUCE
1. Open system settings
2. Go to one of the aforementioned sections
3. Enter the "new stuff" dialogue

OBSERVED RESULT
previews of listed themes load in about 5-7 seconds each. Moreover, when you scroll down and back up, previously loaded images have to be reloaded with same waiting time.
Just to give you an idea, it's like I'm back in the 90s with a 56k connection.

EXPECTED RESULT
Images should load as fast as my internet connection is (it is, a lot). Also notice the old looking dialogue from the "get new window decorations" does not suffer from this issue and loads everything instantly.
I also have a kubuntu 20.10 install (plasma 5.19.5) lying around and it doesn't suffer from this issue in any section, even the new looking dialogues work correctly.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Fedora 5.10.9-201.fc33.x86_64
(available in About System)
KDE Plasma Version: 5.20.5
KDE Frameworks Version: 5.78.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
I've noticed the following interesting lines (hundreds of) in my system log:

gen 31 12:42:04 localhost.localdomain systemsettings5[119631]: file:///usr/lib64/qt5/qml/org/kde/newstuff/qml/private/entrygriddelegates/FeedbackOverlay.qml:34:9: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }
gen 31 12:42:04 localhost.localdomain systemsettings5[119631]: file:///usr/lib64/qt5/qml/org/kde/newstuff/qml/private/entrygriddelegates/FeedbackOverlay.qml:34:9: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }
gen 31 12:42:04 localhost.localdomain systemsettings5[119631]: file:///usr/lib64/qt5/qml/org/kde/newstuff/qml/private/entrygriddelegates/FeedbackOverlay.qml:34:9: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }
gen 31 12:42:04 localhost.localdomain systemsettings5[119631]: file:///usr/lib64/qt5/qml/org/kde/newstuff/qml/private/entrygriddelegates/FeedbackOverlay.qml:34:9: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }
gen 31 12:42:04 localhost.localdomain systemsettings5[119631]: file:///usr/lib64/qt5/qml/org/kde/newstuff/qml/private/entrygriddelegates/FeedbackOverlay.qml:34:9: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }