Latest version of Safari Tech Preview (Release 26 (Safari 10.2, WebKit 12604.1.12)) seems to beachball constantly. Most noticeable on Facebook.com, load the site, try to scroll and it freezes with a beachball. I tried to catch a sample, but couldn't.
<rdar://problem/31322167>
I can reproduce in STP 26 but not with the latest WebKit so this may have been fixed already.
This progressed at https://trac.webkit.org/changeset/214333/, so likely caused by https://trac.webkit.org/changeset/213712
Quick aside.. Simon, who worked on the FilterTop project several years ago? Hi!