No matter what caused the data source to be slow (the old technology, performance issues, slow connector, limitations, etc), it will cause the data refresh of the Power BI dataset to become slow. Even if you have an incremental refresh setup, it might not still help much, because sometimes the query folding doesn’t happen. Slow refresh time will not only be bad for the service, but it will be also bad for the developer who has to wait a long time for the data to be available after each refresh.
Read on to see how you can use Dataflows to speed up refresh times (though not speeding up the slow data source itself). Reza also has a video on the topic.