Instrument Rapid Diffs performance

We should measure the client-side performance of the Rapid Diffs from the get go. Metrics to collect:

1. Regular page speed metrics like LCP, TTFB, TBT, etc.
2. Time to first diff (i.e. first diff visible). Should be measured from the Rails rendered markup.
3. Time to last diff

We might want to utilize 10K SiteSpeed architecture that we have for that.