Matt, thank you very much for your hints. I deinstalled the driver, console and effects with the supplied software. Then manually removed the dspMixFx preferences.
After that, I reinstalled everything, and updated the Yamaha driver (the one included in the Tools 2.2 is outdated).
No difference. dspMixFx continues to start very slowly – from 15 to 55 seconds, depending on the tries.
Paolo