Bug 1536174 added Gecko support for capturing rendered frames: the composition recorder. It's unclear what the performance overhead is of such recording. This ticket tracks using the small framework for capturing granular perf stats with low-overhead added by Bug 1553254 to measure that overhead.
There are two interesting things I'd like to see from this measurement. Obviously, I'm curious to know what the measured overhead is. But we might also be able to see a larger overhead across compound actions, for example, across an entire pageload. That might suggest the actual impact of reserving memory for the frames captured by the composition recorder.