SUMMARY
Taking many screenshots in a row will basically make Dolphin, Gwenview and possibly more programs unusable. I did this as a test, I know it's not normal to mash the screenshot key, but it's still a problem I guess.

STEPS TO REPRODUCE
1. Set a keyboard shortcut key for Capture Active Windows
2. Mash said shortcut a few times (I guess 10 or so)
3. Try to do anything in Plasma, such as starting Dolphin, everything will be delayed and unusable. 
4. Also, if you open System Monitor, Spectacle will be using 20 to 30% CPU without actually doing anything. 
5. Dolphin and Gwenview (possibly more) will become unusable until you kill Spectacle. Then you will also have to kill Plasmashell because that will become unusable too, either that or wait until it crashes on its own and restarts as well. 

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: KDE Neon User
(available in About System)
KDE Plasma Version: 5.18.3
KDE Frameworks Version: 5.67.0
Qt Version: 5.14.1

ADDITIONAL INFORMATION

This is the Plasma crash log after it dies when you kill Spectacle
Thread 1 (Thread 0x7f867563c800 (LWP 8104)):
[KCrash Handler]
#6  0x00007f8672ac28e4 in QV4::WeakValue::markOnce(QV4::MarkStack*) () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#7  0x00007f8672b3b1ec in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#8  0x00007f8672b3b1f7 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#9  0x00007f8672b3b1f7 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#10 0x00007f8672b3b1f7 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#11 0x00007f8672b3b1f7 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#12 0x00007f8672b3b1f7 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#13 0x00007f8672b3b1f7 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#14 0x00007f8672b3b1f7 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#15 0x00007f8672b3b1f7 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#16 0x00007f8672b3b1f7 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#17 0x00007f8672b3b1f7 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#18 0x00007f8672b3b1f7 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#19 0x00007f8672b3b1f7 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#20 0x00007f8672b3b1f7 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#21 0x00007f8672b3b1f7 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#22 0x00007f8672b3b29b in QV4::Heap::QObjectWrapper::markObjects(QV4::Heap::Base*, QV4::MarkStack*) () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#23 0x00007f8672a45b7f in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#24 0x00007f8672ac2228 in QV4::PersistentValueStorage::mark(QV4::MarkStack*) () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#25 0x00007f8672a45f4d in QV4::MemoryManager::collectRoots(QV4::MarkStack*) () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#26 0x00007f8672a4613c in QV4::MemoryManager::mark() () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#27 0x00007f8672a47f95 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#28 0x00007f8672a4a116 in QV4::MemoryManager::allocString(unsigned long) () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#29 0x00007f8672aab83a in QV4::ExecutionEngine::newString(QString const&) () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#30 0x00007f8672af01ac in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#31 0x00007f8672ab2e2f in QV4::ExecutionEngine::newTypeErrorObject(QString const&) () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#32 0x00007f8672ab52b2 in QV4::ExecutionEngine::throwTypeError(QString const&) () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#33 0x00007f8672ac3224 in QV4::Lookup::resolvePrimitiveGetter(QV4::ExecutionEngine*, QV4::Value const&) () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#34 0x00007f8638059500 in  ()
#35 0x0000000000000000 in  ()