Sorry for the long silence. We are actually in the process of writing up a short manuscript summarizing the various problems/lessons learned when running online experiments through MTurk. Part of that will focus on this frame rate issue. I will add a link when it gets published.
In our case a big factor in the low frame rates was that over 80% of workers/participants had hardware acceleration disabled. We added reminders to enable hardware acceleration in the description of the MTurk HIT and added multiple checks inside the experiment that would prompt participants to enable it - clearly without success.
The default renderer of the PsychoPy version we used (2022.2.4) does not perform well when it runs on the CPU, hence the low frame rates. I also think that we did not notice it as much when running previous versions of PsychoPy, as the frame rate calculation was not as accurate.
The PsychoJS development team was very helpful and provided us with a beta build of PsychoJS that detects if hardware acceleration is enabled or not (that’s how I can now say that 80% of participants had it disabled). If disabled, PsychoJS switches to a different renderer that runs more efficiently on the CPU. This increased our frame rates substantially, to the point where HW acceleration off and on are comparable in terms of frame times. If I understood it correctly, this feature should make it into the main PsychoJS branch in the future.