AE 2026 hangs when selecting multiple layers and keyframes

I have used After Effects for over 10 years. Over approximately the last two years, a recurring issue has made basic timeline editing unreliable. It still occurs in After Effects 2026 on Windows. This workflow did not previously cause this behaviour.

Steps to reproduce in an affected project:
1. Open a heavier composition containing multiple animated layers.
2. Select multiple layers, particularly all layers in the timeline.
3. A blue spinning busy cursor appears and persists. After Effects remains partially responsive, but interaction becomes difficult and the cursor keeps spinning.
4. Select a block of keyframes with the intention of copying and pasting them further down the timeline.
5. After Effects hangs and the edit cannot be completed.

Expected result:
Layer and keyframe selection should complete, the busy cursor should clear, and normal copying and pasting should remain possible.

Actual result:
Selecting layers leaves a persistent busy cursor and partial responsiveness. Subsequently selecting a block of keyframes makes the application unusable. The problem begins during selection, before I can complete the intended copy-and-paste operation.

Frequency and scope:
This does not happen in every project. It usually affects heavier projects with multiple layers, and the sequence above consistently triggers the problem in affected projects. I have been encountering this behaviour for roughly two years, and it remains present in AE 2026.

Environment:
After Effects 2026 on Windows. Exact AE build, Windows version, hardware specifications and GPU driver version are not included in this initial report.

Impact and request:
This is a serious, recurring disruption to a fundamental animation workflow. After more than a decade of using After Effects, it is extremely frustrating that selecting layers and keyframes now prevents me from carrying out routine editing in affected projects. Please investigate this issue, confirm whether it matches an existing tracked bug, and provide a reliable workaround and an update on a fix.