How to enable GPU processing for preview playback?

[Elusien]

The GPU is not used when previewing. Dan would like to do it, but it isn’t easy and so would take some effort to do. See here:

Clarification on what "GPU acceleration" means

You already have most areas covered by your numbered list. However, there is one more area, and some reorganization: Decoding could be GPU-accellerated but is not currently in Shotcut. It is difficult when dealing with multiple video tracks, but I want to eventually bring it to the Source player and bottom video track. Another difficulty is to convert the decoded output to OpenGL texture for subsequent processing without making the trip through RAM and then doing this for the variety of APIs …

To get smoother previewing (but slower) you could switch off the setting:
“Realtime (frame dropping)”