While investigating WT-10435, I noticed that the performance of test/format with mirroring not enabled regressed. This was done by comparing running with the config from WT-10435 (see the comments) with commit ffba1bcf0f15a28607c3b664d63d730149c1b3e and then with the mirror commit (addbbad09e823a5c1ed8407eb1e7b861e2bead6e).

I only compared x86 and zseries and found interesting results (average from 5 runs):

Pre mirror (x86): ~262 seconds

Pre mirror (zseries): ~130 seconds

Post mirror (x86): ~197 seconds

Post mirror (zseries): ~168 seconds

Yes it got faster on z86 but slower on zseries. More tests are needed but on zseries it was the difference to hitting WT-10435 or not (runs.timer=4 was set instead of runs.ops).

Might want to check other platforms as well.