Speed is not whats important with an enlarger lens, its resolution. Finding the "sweet spot' of a particular lens is what you are looking for. Stopping down two to three stops, or more is going to give you the balance of edge to edge coverage without vignetting, as well as the proper speed for a good exposure. You dont want to only flash the paper for the least amount of time, conversly, you dont want to have to wait forever for the enlargement. Of course, there are variables, such as extending times for dodging, or split filter printing to adjust the contrast. I recommend finding some good books on darkroom techniques, such as any of the Kodak workshop series. I have "Black and White Darkroom Techniques", and I have "Photographic Printing" by Ralph Hattersley. I'm sure there are many others that can be recommended by other members.
Hope that isnt confusing, and helps.
Rick