Hi Jens!
I’m aware of macs having introduced some form of strange screen refresh timing (my best guess is that it now has a system of “virtual” frames like its system of virtual pixels in the retina display meaning that what you get is what MacOS thinks you probably want, rather than what you explicitly request). We’re currently exploring how to get around this on the Python side but I haven’t tested or explored the problem on PsychoJS yet. Getting control of this through the browser could be even more difficult.
But I think it’s fair to say that on MacOS timing by frames is currently not possible 
Best wishes,
Jon