I am filing this bug as reported in the 2d-dev list by [~alans]

https://mail.openjdk.java.net/pipermail/2d-dev/2021-April/012815.html

Text from the mail :
---------------------------------------------------------
I am seeing some unusual behavior (in b20) that I do not see using OpenGL (or using JDK 16).

Sometimes when I open a new window, the window appears blank (except for the title bar) for about two seconds before the content appears.

This behavior is not consistent. Opening another instance of the same window might be fast or slow. It happens with a variety of window classes.

In JDK16 and using OpenGL, the content always appears immediately.
-----------------------------------------------------

More details are at -
https://mail.openjdk.java.net/pipermail/2d-dev/2021-May/012818.html

In JDK17 b20, we have metal rendering pipeline as the default pipeline on macOS. Hence, this observed behavior might be related to metal rendering pipeline.
Note : We will need a test program to see this behavior. There is no test program provided.