Good news. What was by far the main performance problem on X11 is now fixed in the latest commit: "Fixed a bug in the rectangle painting-tree".
It was a regression we introduced a couple month ago when optimizing PDF rendering. What this regression did is turn off the optimization that when a view totally covers another view, then only the toplevel one gets drawn. Because of that regression, in Jedi when you would had 10 documents opened, then each drawing operation, selection, scrolling, ... would end up drawing the 10 documents!!