Richard, it's remarkably easy and accurate to test the slow speed, all you need is a microphone and a program like Sound Forge or better still Audacity which is free.
Set the microphone and Audacity to record then cock the shutter and trip it at 1 second, then 1/2, 1/5, 1/10, etc then when you analyse the sound wave you can see the spike where the blades open & then later close, you zoom in and read off the time-line.
Looking at my 1933 Compur it takes less than 1/100th second for the mechanism to open the blade, and a fraction less to close them, but I can accurately measure how long they are open for in between and in my case despite it's age the shutter runs a fraction fast, they normally begin to run slower at they age.
You can hear the speeds here
Compur. Accurate is within 15% of the marked speed.
As my most common speeds were generally below 1/30 this is a good test, it certainly tells you when speeds are drifting off. My newer Copal shutters inc very recent Copal 0 are a touch slower but then Copals are not as good as even pre-War Compur's in terms of long term reliability, even my dial-set Compurs are accurate.
Ian