Hi Marcel,
I've got now an appointment for a remote session with IDrive. I will keep you updated.
Interestingly, I figured out that if I create a "Backup set" for every subfolder (there are 7), the upload speed increases to ~700kB/s. For me it looks like, that IDrive caps the upload speed at 100kB/s per connection.
Probably I will now get the standard reply, that IDrive doesn't cap the upload speed. But now I'm really interested, how you can explain this!