Hi Cheryl,
You have configured the time slider correctly for cumulative display, and the 'blinking' effect is (unfortunately) a result of how long the updated filter is taking to update the view. That is, Pro is pulling the updated subset of features (based on the time slider change) and is not drawing them fast enough to avoid the 'blink'. You can try reducing the playback speed - which will show each time slice a little bit longer before pulling the next time slice - but you will likely still see a similar effect.
Another approach, useful for sharing out a time-aware playback video, is to create an animation. This will NOT improve interactive playback within Pro - the time the view needs to filter and re-render a time slice will remain the same - but it does give you the ability to export the playback to a video file. The video will be smooth because each frame is fully rendered before being exported.
Here's a help topic link that can get you started on creating a time-aware animation:
- Animate through time—Animation | ArcGIS Desktop
Quick tip: you will most likely only need to create two keyframes (one with the time-slider set to 1890-1891, and one with the time-slider set to 1890-1971), update the video duration (default will be 3 secs, but you probably want a 10-15 second playback time), and then export it to a YouTube-friendly video file.
Hope this helps!
Thanks, Nathan.
ArcGIS Pro team