Even if Progress Bar was stopped (suspended), CPU resource was exhausted.
I could see this issue with High Sierra, Mojave, Catalina.

Test instruction is as follows:
1. Start "Activity Monitor" via Finder
2. On "Window" menu, select "CPU History"
3. Start java with SwingSet2 demo
4. Please wait until CPU usage is stable
5. Click JProgressBar Demo icon on JToolBar (9th icon from left side)
6. Press "Start Loading Text" button, and wait a few seconds
7. Press "Stop Loading Text" button
8. By step 6, CPU usage becomes HIGH, by step 7, CPU usage should be becomes LOW
9. On "Look & Feel" menu on SwingSet2, select "Java Look & feel", CPU usage becomes LOW
10. On "Look & Feel" menu on SwingSet2, select "Mac OS X Look & feel", CPU usage becomes higher

Same kind issue was reported by JDK-8147843, but I think usage was not same.