Investigate DM QA processing speeds

Arnaud Le Fur:
Those are single visit, eg no CR split, so no impact from those updates.
It's also awfully slow using w.2026.01 from the command line, just reducing 136517 only b,r arm were used.
pipetask --log-level .=INFO run --fail-fast --instrument lsst.obs.pfs.PrimeFocusSpectrograph -b /work/datastore -i PFS/defaults -o u/alefur/w.2026.01/test1 -p "$PFS_INSTDATA_DIR/config/pipelines/hilo.yaml" -d "visit IN (136517)" -j 16 >> 136517.log 2>&1
about 350s goes into reduceExposure, 36s in mergeArms, about 200s in dmResiduals, 90s into dmCombinedResiduals
~11minutes