High CPU load when playing through long gap between subtitles

[rndusr]

How To Reproduce

1. Place point on a subtitle that doesn't have a following subtitle for a few
   minutes.

2. M-x subed-enable-sync-point-to-player

3. Let the video play.

4. After a while (tens of seconds in my case), CPU load on a single core spikes
   to 100%. Eventually, video playback becomes choppy and Emacs takes several
   seconds to respond to user input.

What I Know

The relevant code seems to be in subed-srt--subtitle-id-at-msecs, specifically
in the final (catch 'subtitle-id ...). If I comment that out, the issue goes
away, but I don't understand what's wrong with the code or how to investigate
further.