Here we go again with the "Leaf shutters are not as fast as they are marked" crap.
A properly serviced leaf shutter should be within 1/6th stop at all speeds. Where this they don't work as advertised comes from is people checking out old gummed up shutters, and people using a cheap shutter speed tester incorrectly.
Speed of leaf shutters is measured at the 70% open point. It takes a shutter about a milli-second to reach that point and another milli-second to finish closing from that point. That is 2 milli-seconds. Now a 1/500 second shutter speed is open for 2 milli-seconds, that is a total of 4 milli-seconds from start to finish. If you do not fallow the written instructions with your Calumet, or whatever, shutter speed tester you will get a reading of 1/250 second. But if you use a dimmer on the light so the tester does not start counting until that 70% open point then you will get a reading of 1/500 of a second, as you should.
Very expensive shutter speed testers, like the shutter and lens makers use, do all that automatically, but not many, if any, of the under $1000, much less under $100, do so. With my cheap Calumet tester, every properly working shutter I have tested, doing it correctly, has been within 1/6th of a stop at all shutter speeds.
Why do folks not see that error at slower speeds? No matter what shutter speed you have the shutter set for it is still going to have that 2 milli-second of lag, only at 1/100 sec that is a 2% error, hardly noticeable. With a 1/1000 sec shutter it is a 200% error in your reading (except that Graphic lens gets it speed from having less lag, and you will see only 100% at most.)
An easy way I use for quick shutter checks is to simply subtract those 2 milli-seconds from the reading on the tester, that gives an in the ball park shutter speed that is close enough to tell you if the shutter needs servicing.
If your shutter is actually a stop or two slow at its top speed, it needs servicing.
I am tired of this stupid old myth!