Just tried it out, front-end performance is not a concern at all 🙂 although speeding up the initial API response that gets the discussion and its list of post IDs might be something we need to look into a bit later.
In my mind the scrubber is for jumping to an approximate position in the discussion — kind of like pagination. In a discussion with 14k posts this is obviously not very precise (see the GIF below), but using the new "true" infinite scroll you can very quickly scroll to where you want to be.
[attachment:5568efb557216]
You can also jump to an exact position in the discussion by changing the number in the URL. If there is enough demand, we could consider slipping a "go to number" field into the UI.
