What speed are you connecting to the other fax machine at?
The maximum fax speed that WinFax will support is 14,400 bps but not all fax machines will support this. Some older machines connect at 9,600 bps only and if you have line noise or other errors the connection speed can drop to 7200,4800 or 2400 bps.
It also depends what you’re sending, if your fax pages contain graphics (a lot of black pixels) then your document will take much longer to send compared to a page with few lines of text and a lot of white space.
On average, a 2 page fax should take approximately 2 minutes to send under normal 9,600 bps transmission speed. A bit longer for higher quality graphic pages.
Its also possible you have ECM turned on, and fax pages are failing and being re-transmitted causing your lengthy transmission times.