I have not actually noticed too much about the performance. But it's definitely not super fast. Sometimes it will pause to "avoid ... restrict". By design, it's an offline downloader that not take performance as the first consideration.
I downloaded 10 forex 1min data for a month, took approximately 5 minutes maybe? Most of the time it is just waiting.
Just noticed that Trading Geeks is offering 15 dollars discount until end of June on their IB Data Downloader. http://www.tradinggeeks.net/downloads/ib-data-downloader/