It has always been one stop faster than tmax 400 for me. I do have lots of examples of fogged (aged) film showing a loss of speed as it gets older. Maybe you are testing old fogged rolls of TMZ.
I remember back in 1988 using TMZ for a college reunion and being under the impression the rolls I got from B&H had been sitting around for a long time on the shelf and were pretty fogged. I suspect the situation may be worse now, but it has been a few years since trying it. Maybe I should get some more and see.