Last night I set up a render queue with about a hundred comps that are basically the same (a 1m30s text animation, the actual text is only thing difference between the comps). First 50 took about 4 to 5 minutes per comp to render, then it suddenly went up, now taking over 50 minutes for the same kind of comp. There is a green part in the renderbar and the render is still progressing (sometimes showing a lightgreen part in the bar), but checking the info it says 'Concurrent frames rendering: 0 '. This seems incorrect.
So my guess is that MFR is where it broke down somehow. But maybe it's something else, I don't know. Anyway, it's now much slower then rendering without MFR (it will render a comp in just under 10 minutes without MFR), defeating the purpose of MFR.
At least for now using MFR on a render queue seems unreliable.
I'm running AE 24.1.0 on a M2 Ultra Mac Studio, 128GB RAM, macOS 13.6.3.
Likely a situation where the memory has become exhausted and we've found on the M2/M3 machines, the OS is much less forgiving about recycling memory. That said, we've made some improvements in the current 24.2 beta so you could try installing that and then see if the problem continues.