Judging by my experience with all kinds of old and not-so-old cameras, usually the average mechanical shutter works well within in 1/3 - 1/2 stop from the expected value. I measured it on tons of cameras with a digital storage meter (a Soviet one, made on radio tubes ). The error is usually non-systematic (e.g. 1/30 can be slower, say, 1/25 - but 1/125 is something like 1/150), especially in older leaf shutters. It all can be cured during a good CLA - in leaf shutters by filing down or delicate squeezing out the worn profile on a speed-setting ring. As far as I can remember, 1/3 of stop was a factory tolerance in both GDR and USSR, but the new specimens were usually close to set values. It shouldn't bother anyone, though - except ZS people, maybe.

Cheers from Moscow,