I confirmed this several time already, with manual jobs and scheduled jobs, always the same behavior (using latest version of the software, btw). What happens is that the backup process will be running smoothy and transferring at top speed (10MB/s on a 100Mbps connection), then it nosedives to 1MB/s just by closing the GUI (SBF.Application). Re-opening the GUI immediately makes the speed rise to the top within 5 seconds.
To investigate this case we need more details. Could you please enable the advanced log, reproduce the issue (when everything works smoothly and when a back job takes much time to perform a backup) and sen the logs to our dev team. Here are more detailson on how to do it: https://sqlbackupandftp.com/blog/how-to-send-log-to-developers
Please let us know when the logs will be sent, also please provide us with your Application ID (“Help” > “About”).