Hi David & Richard,
OpenJDK 17 Early Access build 18 is now available at https://jdk.java.net/17
G1 pauses may be extremely long with EA build JDK-17+18
During performance testing we noticed that due to a recent change (JDK-8262068) GC pauses after a G1 full GC may be extremely slow. The problem has been fixed with JDK-8264987 and that has already been integrated. This change will be available with the following EA build JDK-17+19. For more technical info please see 
JEP 382  - Starting with build 19, JDK 17
for macOS is *temporarily* switched from using OpenGL to
using Apple's Metal API for Java 2D rendering.
-- Rgds, Rory O'Donnell Quality Engineering Manager Oracle EMEA, Dublin, Ireland