First of all, thanks for the quick reply.
With the chunk size we have actually tried everything from 1-250MB, but thanks.
Since we have such speed problems neither with Google drive nor Google cloud storage nor Azure (there we have GBit without tweaking) nor Backblaze (there we also have GBit upload, but only with: --transfers 5 --b2-chunk-size 600M - -b2-upload-cutoff 600M), it will probably be due to Ondedrive.
It's actually a shame that MS obviously wants to urge its business customers to handle everything via Azure.