If you are using version 26.1 try turning off Settings > Preview Scaling > Use Hardware Decoder. Otherwise, preview scaling without making proxies at the same resolution can actually make things slower because it needs to scale the source to the preview resolution, and scale = work.
Convert the Video into Proxy… Maybe, and most likely, Convert the Original Video into Edit friendly format. I had issues same until deeper research via AI and found this:
How to Do It in Shotcut:
Import your video.
When prompted, click “Convert to Edit-friendly…”
Or right-click the clip → Properties → Convert to Edit-friendly.
Choose a preset:
Good: H.264 (smaller, decent performance)
Better: ProRes/ALAC MOV (high quality, larger)
Best: FFV1/FLAC MKV (lossless, largest)
This conversion ensures smooth editing, especially with screen recordings, TikTok live videos, or phone footage. The size may turn out to be bigger than original video (keep in mind).
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NOTE: converting to an edit-friendly format in Shotcut. This improves editing performance by changing poorly optimized or variable frame rate (VFR) videos into constant frame rate (CFR), intra-frame encoded formats that are easier for your computer to decode in real time.
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Alternative, if Video Size is an issue, to use FFMEG.
”FFmpeg is a powerful, cross-platform command-line tool for converting video and audio files between various formats.” AI can give you command line Prompt to convert Video Files to the same Friendly Edited Version that Shotcut gives Automatically, but smaller file size. Do further research on the keywords.