I recently upgraded this server to Ventura 13.6.3 and FileMaker Server 20.3.1.31 and have re-run the test.
- FMS 19.6.3 (Server Script), M1 Ventura 13.5: 51 seconds
- FMS 20.3.1.31 (Server Script), M1 Ventura 13.6.3: 50 seconds
So, updating to FMS20 does not seem to help; whatever slowdown was introduced between FMS18 and FMS19 seems to persist in FMS20.
I'm still seeing the same pathological behavior, where both the FileMaker Server process and the FileMaker Scripting process never use more than about 20% of the CPU.