Every Synchro Compur I've had has had sticky slow speeds.
With most of them it has been a bit of oil on the blades and a gentle wipe with a tissue moistened with lighter fluid followed by firing the shutter and gently wiping dry.
This will get the oil of the blades and often makes the shutter come back to life.
They are reliable old things.
I know this is not the completely correct technique nor is it a replacement for a full CLA but it has worked for me on half a dozen cameras which were in good condition but had slightly sticky shutters.
Gentle touch is required though.
Also sewing machine oil isn't really the best...there are some synthetic watch oils that do an excellent job but it's ten times the price of sewing machine oil.