The attached test program demonstrates the problem on my system, a high end iMac running macOS 11.5.1. When the window first appears, it contains no content. The text appears about one second later (actually 0.78 seconds according to the screen recordning). I believe it is inappropriate to ever display a window in an incomplete state. A delay of one second is noticeable even when the change is subtle.

I tried the Metal pipeline. It might be a tad slower.