The speed of the payment sync is controlled by the Sage SDK. We suggest trying a Custom Date of one day to see if that helps with the speed. The integration has to look at (1) all payments within the requested date range, and then (2) for every payment founds within the date range, the integration needs to search all payments within Sage to see if there is another partial payment for that invoice. As a result, the wider the date range, the longer the integration may take, depending on the size of your Sage database.
So, the sync speed is limited to the speed of the environment and the Sage SDK, but troubleshooting with a newer, smaller company file and/or a local copy of the data file might give your IT team a better idea of what might be slowing the process down.