I just counted: in one window where I'm experiencing laggy control selecting, dragging, resizing, deleting, etc I have 48 controls. This is pretty standard.
In a Preferences window, I have 263 controls. Here's the strange part — both windows have the same exact (lack of) responsiveness, delay, and lag. There's no discernible difference.
When I create a brand new blank project, dragging controls is quite fast - almost zero lag. As I start copying and pasting controls, once I hit about 25, it becomes noticeably slower. Then I continued adding controls until I had over 250, and the lag barely increased. So basically, 1-20 controls is very responsive, after 20-25, I get noticeable lag, which remains consistent through upwards of 300 controls. I haven't tested yet with more than 300 controls. But It seems there's a threshold of about 20 controls before the lag hits.
Opening my window with 48 controls initially in the IDE is pretty fast, 1-2 seconds of delay, and opening the Preferences window in the IDE has about 4-5 seconds delay. But once they're both open, they are similarly responsive (or not responsive, as it were).