We've found that our unit tests (a few hundred .t file, 9285 individual tests) which take 23.5 minutes on a 3Ghz Xeon box with 1Gb of RAM, run in 12 minutes on a P4 2.8 HT box with 640Mb of RAM. When I get done with some performance optimizations, I hope to get that under 10 minutes.
The Xeon box has SCSI disks; the HT box has IDE. A large number of the tests involve traffic to Postgres, running on the same box. Both boxes run RedHat 8. We'll be testing later on one of the Fedoras.
Unfortunately, I'd just ordered up parts for a non-HT P4 2.8 box for home. :(
