One thing that most of these types of testers don't do is measure a leaf shutter at 70% open as per the spec's. That makes it look like the shutter is running slow at higher speeds. That is where the stories about leaf shutters only doing 1/300 of a second at 1/500 come from. All of the properly CLA'd shutters (post wwii) I have tested have run within 1/6 of a stop at all speeds. If it does not, then it needs to be properly serviced.
I have one of the Calumet Shutter Speed Testers and the instructions tell you to use a dimmer on your light source to make it read that 70% open speed. I do not know it you can use that technique with the sellers testers or not.