Solved it:
First (1 week ago) I copied the library on a (USB3) disk which was read and write enabled, that made reading the library a little bit faster, even though still on a USB2-port.
Second (today) I installed a Sonnettech Allegro Pro USB 3.0 PCIe-card in my MacPro 5.1, that made the reading-times of my library back fast again.
Niek/ Amsterdam.