Well, I tried my shutter at 1/10, and guess what? At least with 127mm Ektar lens everything is sharp, without a single trace of shake blur! I was right, and you, too - the curtain clunks when it's stopped, but the frame is closed by the time so no blur can occur I am going to adapt many barrel lenses to my Speed, because my dear friend Mario can make to me as much good metal lensboards as I wish
Cheers, and thank you for your replies! I'll post my pictures soon!