Greetings. I work on several projects and on two different computers same time and to keep latest sources I use my flash memory stick. However, some time, when I click to save the project it takes some several minutes! Project is not heavy and there is no large picture files etc. This does not happen always. Is there any suggestion about what could go wrong?

I suspect this has very little to do with SiteSpinner and a great deal to do with Windows.
When slow, is it slow only on one computer? Do you get similar results when saving to the hard disks -- allowing for the fact that the hard disks are probably inherently faster?

Yes, I have faced same problem with different computers and on flush stick however not noticed ever with saving on hard disk.

USB flash drives (memory stick, smart card, etc) are slower than hard disk drives because they are re-writable memory chips. Every time you want to save something on a flash device, whatever is there must be erased/cleared before the new data is written. This is what makes them slower than hard drives (which only change magnetic state). Also be aware that these devices have a limited number of times they can be written to before they start to degrade (a few years ago the limit was 10,000 "burns"). You should be able to speed up your save by re-formatting the device before use, but the reformat time will negate any time savings.