Bug 513263

SUMMARY
Timeline navigation becomes noticeably slow when the number of clips reaches ~800 or more.
Scrolling and zooming the timeline lag, making editing uncomfortable even without effects,
transitions, or animations.

STEPS TO REPRODUCE
1. Create a project with long footage (e.g. ~1.5 hours) on two video tracks with two synced audio tracks.
2. Perform extensive cutting to remove unwanted parts until the timeline contains ~800–1000 individual clips.
3. Try scrolling horizontally and zooming in/out on the timeline, especially during playback.

OBSERVED RESULT
- Timeline scrolling becomes sluggish.
- Timeline zooming lags and feels delayed/step-based.
- While playback is running, scroll and zoom responsiveness degrades further.
- The UI remains functional, but navigation becomes uncomfortable and slow.
- System resources (CPU/GPU/RAM) are not fully utilized.

EXPECTED RESULT
Timeline scrolling and zooming should remain responsive with hundreds of clips,
especially during basic rough-cut workflows without effects or compositing.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Ubuntu 24.04 (Wayland, GNOME)
KDE Plasma Version: N/A

ADDITIONAL INFORMATION
- I will provide a download link to the project file so the issue can be reproduced directly.
- Actual Kdenlive version: 25.11.70
- Project link download: https://drive.google.com/file/d/1bXhkKy3dZVdrqdJYDd5acxvI1uIT6S2F/view?usp=drive_link