Timeline UI gets progressively slower as more keyframes are selected with marquee selection

When selecting a large number of keyframes in the Timeline using marquee selection, the After Effects UI becomes progressively slower as the number of selected keyframes increases.

This is especially noticeable when repeatedly adding more keyframes to the current selection.

Steps to reproduce

1. Create a composition with a large number of keyframes across one or more properties/layers.

2. In the Timeline, use marquee selection to select some of the keyframes.

3. Keep adding more keyframes to the selection using additional marquee selections.

4. Observe the Timeline UI responsiveness as the number of selected keyframes increases.

Actual result

The Timeline gradually becomes less responsive as more keyframes are selected.

In my case, the slowdown starts becoming clearly noticeable at around 100 selected keyframes. As the selection grows further, marquee selection and other Timeline interactions become increasingly sluggish.

The important point is that this does not appear to be caused simply by having many keyframes in the composition. The performance degradation appears to correlate with the number of currently selected keyframes.

Expected result

Selecting additional keyframes should not cause Timeline UI performance to progressively degrade based on the number of already selected keyframes.

Selection and marquee interactions should remain reasonably responsive even when hundreds of keyframes are selected.

There have been previous reports of Timeline performance issues involving large keyframe selections, but I could not find a report specifically describing this behavior where the UI becomes progressively slower as the current keyframe selection grows.