I want to point out that I noticed this problem with 5.5 when I was using Console Command.
Opening some widget windows locks the framerate of the viewport to 30FPS with no option to disable It.
For example: Game Animation Sample.
Project offers a widget to provide additional settings to tweak experience.
This happens when some widgets are open. When It’s closed (or docked unopen) It will restore frame rate.
Edit:
No, option “Use Less CPU in Background” was not enabled