Resizing a window with a single way view performs good enough, but when adding more tabs, the performance decreases a lot. This is because resize is a sync operation, and the UI process waits for the web process to have a new update for the new size, while still draws the previous frame. This is needed for the visible web view, to avoid flickering and artifacts while resizing, but for all other hidden web views, we don't really need to block the UI process. This doesn't happen in Wayland, because in Wayland we never block the UI process while waiting for web process update after a resize.
Created attachment 289569 [details]
Comment on attachment 289569 [details]
Works for me.
Committed r206294: <http://trac.webkit.org/changeset/206294>