Not entirely sure why you specifically ask for performance improvements (I’m not aware of this release bringing particularly many performance improvements).

The changelog for the v89 Beta is here: https://www.mozilla.org/en-US/firefox/89.0beta/releasenotes/

Performance related items as far as I can tell:

We are rolling out Version 2 of our existing SmartBlock feature, which enables a seamless web page experience that would otherwise stop working when we block trackers.

(SmartBlock ensures that webpages don’t wait forever for a blocked resource to load.)

Introducing a non-native implementation of web form controls, which delivers a new modern design and some improvements to page load performance.

There’s obviously also many smaller changes which aren’t mentioned im the changelog, but may affect performance: https://bugzilla.mozilla.org/buglist.cgi?j_top=OR&f1=target_milestone&o3=equals&v3=Firefox 89&o1=equals&resolution=FIXED&o2=anyexact&query_format=advanced&f3=target_milestone&f2=cf_status_firefox89&bug_status=RESOLVED&bug_status=VERIFIED&bug_status=CLOSED&v1=mozilla89&v2=fixed%2Cverified&limit=0

And of course, they’ve tweaked the UI to expose those items better that users click on the most on average, so if you’re not an outlier in that regard, then it may feel faster from that as well.