My best guess is an issue/limitation between your ISP and OneDrive.
Do you see the same speed if uploading through the OneDrive web interface in your browser? (Time the upload of a 100MB file)
Do you see the same speed if uploading through the native Windows OneDrive folder/client?
Do you see the same speed if using rclone (with default settings) to upload the 100 MB file from a testfolder with just this one file, that is
rclone copy ./testfolder YourOneDrivePersonal:testfolder --ignore-times --progress -vv
I don’t think your rclone setup is causing your issue, but I generally recommend keeping --transfers at default (4) with OneDrive to avoid throttling (429/Too many requests). Links to more info here.