I've started a new project and am bringing things in piece by piece. I believe I have found (one of) the main issue(s). I will be reporting via Feedback ASAP.
1. As I add more and more things progressively to a blank project, performance progressively gets slower. There wasn't "one thing" that impacted overall performance.
2. The more tabs and especially windows open, the more performance slows to a crawl. Typing is almost impossible when you have two windows open. When things are kept tabbed in one window, typing only seems to slow down when you're in larger methods, with progressively worse performance the larger the method.
3. This one is HUGE: controls are responsive and can be clicked and dragged in a layout editor easily, until you have NESTED controls. I had a window with 40+ controls and it remained responsive. As soon as I added a small GroupBox and put a button inside the group box, the entire IDE slowed to a crawl, including switching between the code editor, typing in the code editor, and clicking/dragging/manipulating controls in the layout editor.
Make a brand new desktop application. Add a PushButton to the window. Nice and responsive. Drag it around, double-click, add more controls, everything is great.
Now simply add one GroupBox, make it about the height and width of the window, and make sure the PushButton is inside. For me, performance instantly tanks. Dragging is extremely lagged, double-clicking misfires and moves the control around the screen, switching to the code editor and back is much slower, and adding new controls or moving anything around is barely doable.
This is probably the main issue for me. I use a lot of nested controls. I would imagine most people do. And if you have several windows open in tabs, each with nested controls, the IDE becomes very unresponsive. I've included a video showing how extreme the issue is: https://www.dropbox.com/s/sh5jkqmqwlfwfgb/Xojo%20performance%20issue.mov?dl=0