Ruud

Is it possible that this is coincidence?

Windows updates are the bane of my life, but in themselves have not directlt affected upload/download speeds (after restart).

Have you tried alternate FTP clients? If not, export the project to disk, run an FTP client (filezilla is free) and upload your site. If there is no delay, please adivise, but if there is, please contact your hosting service and ask them to check there are no issues.

(you may also wish to test by (temporarily) disabling any 3rd party antivirus)

Of course I tried uploading via FTP Filezilla. But this also is much more slower as before the Windows update.

I contacted the hosting service. But there are no issues/no changes which could cause this problem. 

I do not use any 3rd party antivirus, only the one which Windows provides.

The problem started after there was a Windows update. So this can not be a coincidence, sorry...

Ruud, I can only think of 2 things to try, the first would be (temporarily) disable defender and see if that fixes it, the second (if the wait is intolerable) would be to remove the update (is it the fall update? http://www.thewindowsclub.com/rollback-uninstall-windows-10-creators-update), or if Win10 hasn't deleted them, you could try system restore to an earlier time.

Other than this I do not know what to suggest, it does seem odd that only FTP to your host appears to be affected however.