When I switched to 65 some time ago I thought I saw better fullyLoaded metrics and today I looked into it more and it looks better across the board. I pinged the perftest channel at Mozilla and asked about where I can check for changes between 64->65 (there's nothing that could explain it at https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/65).

Here's a summary:
On my own instance I cannot see anything clearly for first visual change/speed index but fullyLoaded showed a big change for some URLs:

For WebPageReplay I could see small improvements almost on all URLs:

And on WebPageTest the difference is larger (at that moment we where running c4.large and not c5.xlarge):