SUMMARY
I think this is better described as a video: https://youtu.be/bDFsksynmKk
tl;dr:
* standalone window -> effect is smooth
* another window behind -> effect is stuttering

STEPS TO REPRODUCE
1. Enable Magic Lamp effect, optionally Show FPS as well
2. Open two apps, preferably KDE apps (shouldn't matter, but just in case)
3. Minimize both
4. Start maximizing/minimizing one with the other stay minimized
5. Maximize the other
6. Start maximizing/minimizing again with the same window from step 4

OBSERVED RESULT
With Show FPS, at step 4 the fps only goes down by 10 or so and only at the first or second minimize. The subsequent application is stable at highest reachable fps. Visually, the animation is very smooth like there's no frame skipping.
At step 6, the fps goes down by up to 30 with 20-ish average and it stays so subsequently. Visually, the animation is like there are skipped frames so the smoothness is much less.

EXPECTED RESULT
Step 6 should have the same performance as step 4.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Manjaro with 5.9.11-3 kernel
KDE Plasma Version: 5.20.4
KDE Frameworks Version: 5.76.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
* CPU/GPU: i7-7700HQ/Intel UHD 620
* KWin MaxFPS: 144

Compositor:
* Scale method: Crisp
* Rendering backend: OpenGL 3.1
* Vsync: auto
* Allow application to block compositing: ✓

Other enabled desktop effects:
* Accessibility: Zoom
* Appearance: Background contrast, Blur, Desaturate Unresponsive Applications, Fading Popups, Full Screen, Login, Logout, Maximize, Morphing popups, Screen Edge, Sheet, Sliding popups, Translucency, Wobbly Windows
* Focus: Dialog Parent, Dim Inactive, Dim Screen for Administrator Mode, Slide Back
* Show Desktop Animation: Window Aperture
* Virtual Desktop Switching Animation: Desktop Cube Animation
* Window Management: Desktop Cube, Present Windows, Resize Window
* Window Open/Close Animation: Glide