Thanks for the ideas, I will try to measure the speed when I will have the time for it.
At the moment I try to work on local files stored on the MacBook Air, but it cannot hold that much so I have to keep files on external drives. But what has been once moved to the USB drive that is slow to use.
I either move all the files back to the MBA what takes a lot of time - or open the folder containing them on the USB drive what seems also slow...