As _paintLines method does everything synchronously, the order of linerows being processed does not matter. Removing "invisibleLineRow" logic will eliminate calls to scrollTop and offsetTop methods (which force sync relayout).
Created attachment 184182 [details] Patch
Comment on attachment 184182 [details] Patch Clearing flags on attachment: 184182 Committed r140538: <http://trac.webkit.org/changeset/140538>
All reviewed patches have been landed. Closing bug.