Ah yes, Disabling Real-time in the Viewport does not do much for me either. I assume your monitor’s refresh rate is 120hz. (Vsync on by default) So, you may scoff at gimping your refresh rate, but changing it to 60 in your display settings should make UEFN run at 60fps
This is a good example of why we are currently living in the ‘Bad old days’ of UEFN development.
In Unreal Editor (Not UEFN) there is a ‘Used Fixed Frame Rate’ option in Project settings.
In Unreal Editor there is also access to the console (tilde key), and you would be able to use the console command t.maxfps to limit framerate.
So, Until one of those two things makes their way into UEFN, I think changing your refresh rate is the only way.