Define fast.
There are no mainstream powerchairs that are faster than 8.5 mph. (some pride/invacare/sunrise etc) And these tend to lack torque, and eat batteries faster.
There are a couple. A bounder (11.4 mph option is very heavy and dated), lacks torque and range, and the somewhat heavy/wide 15mph chair by Brad Soden the speedster.
http://tankchair.net/store/speedster-product of which I have little knowledge. But it uses a similar electronics setup to most slow powerchairs and the same heavy lead batteries so. So likely lacks torque at that speed/gearing too.
And possibly the segway/various different wheelchair adapted ones do about 12 to 14 mph.
The bounder is heavy. The others are some 28 inches wide.
Then theres my lithium powered 15 mph chair that is light and narrow, and has tons of torque and range. But you cant buy that since its a one off built for me, by me... here:
http://www.wheelchairdriver.com/BM-MK3- ... rchair.htmAs far as I know these are the only faster than 8mph options.