According to my field guide there's about 100 different species of deer fly in North America (plus the horse flies) - were you thinking of one in particular? ;-) I suppose most of them average about the same speed - 9 or 10 mph seems about right.
In central Ontario they don't usually bother me on the road bike but when I ride cottage roads through wooded areas on my mountain bike they provide a lot of incentive for me to maintain my speed up the hills. I wear heavy cotton MTB shorts and T - shirts rather than spandex and microfibre. I dread getting a flat tire! I'll do a little "speed vs bite" survey next time I'm up there.
My wife does exercise walks on the cottage roads and last year I suggested she wear one of those sticky fly patches on the back of her cap. She came back saying it didn't do any good but when she took her cap off there were about 20 flies stuck on the patch. I guess you just need a much bigger patch!