I'm working as a webperf guy as a staff. I find the work interesting (although sometimes frustrating), but I'm not sure how many companies want to have full-time perf people onsite. As you mentioned, It's mostly consulting type of work now as far as I can tell (so might not be suitable to everyone), but maybe this will evolve or is already evolving (I've been only approached once for webperf onsite position on LinkedIn in last year, by Atlassian, but maybe because my profile is not that well LinkedIn-optimized).
Anyway, if someone's interested, you can start by reading articles from:
https://developer.mozilla.org/en-US/docs/Learn/Performance
https://calendar.perfplanet.com/
https://phabricator.wikimedia.org/phame/blog/view/7/
And some of my own content:
https://calendar.perfplanet.com/2018/getting-started-with-we...