Beyond the performance hit, accessibility also nosedived. I recognize most unaffected users don't care and most developers don't care, but I manage a UX team that can't release front end work that doesn't pass WCAG 2.1 AA not just because it makes better websites but also most public-facing work should consider risk mitigation as their default design posture. Obviously you're not a commercial site and aren't concerned about ADA suits but it's pretty easy to use solutions that don't ruin screen readers, kb navigation, or otherwise lock out users who need accommodation.
I'd imagine your browser metrics skew heavily on desktop to Chrome like every other site on earth, so anything negatively impacting TTVR, TTI, TTFI, page load, CLS will be amplified to the end user given how awful a resource hog it is. (Yes, they should get a better browser, no, they aren't likely to.)
Anyways, I enjoy the service the site has provided over the years and will personally figure out how to get by with Tampermonkey. I simply wanted to offer a POV of a user who relies on accessible sites personally and isn't bashing forced change "just because." (And also to support the immediate and clear performance degradation comments of moving away from the legacy reader. Maybe not everyone experiences it, but it's there.)