It looks like we have an error in the patch where we are initializing the secondary thread for all clients
<rdar://problem/60344018>
Created attachment 393295 [details] patch
Created attachment 393299 [details] Patch
Comment on attachment 393299 [details] Patch Clearing flags on attachment: 393299 Committed r258297: <https://trac.webkit.org/changeset/258297>
All reviewed patches have been landed. Closing bug.