Stuttering when another window is open

[leledumbo]

I think this is better described as a video: https://youtu.be/bDFsksynmKk
The video is actually recorded with KWin's Magic Lamp effect, buat Yet Another Magic Lamp also suffers from the same issue.
tl;dr:

• standalone window -> effect is smooth
• another window behind -> effect is stuttering

Additional info:

• 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