(In reply to Jim Mathies [:jimm] from comment #1)
Worth investigating to understand more but not worth blocking on. It might turn out to be general slow down on lower end systems simply trying to keep up with more active child processes.
In that case, I will move this bug to Fission Milestone "Future" so it doesn't block shipping Fission MVP. We will still keep an eye on CONTENT_FRAME_TIME_VSYNC in our Beta 90–91 experiments and Release experiments.
Comparing our Beta 89 experiment's user segments for high/low CPU and high/low memory, it looks like CONTENT_FRAME_TIME_VSYNC is generally better with Fission, but users with low memory (<= 4 GB RAM) are more likely to have longer long tails. The CONTENT_FRAME_TIME_VSYNC regressions seem more correlated with low memory than low CPU.