Groups are fluid, members change and so does speed. Groups that at one time were crazy fast may become leisurely and vice-versa. I believe what is paramount is to find a group that caters to your needs in training and in camraderie. It may be that Britton's Ladies only ride is the one for you. Or it may be that you like the STORM Thursday Night stay-together ride that is crazy fast that starts at Broadway Bike World around 7:30.
Of course the fastest I know are the ones that smoked me so bad it left me spinning arond in circles at the intersection. The first time this ever happened was when I had recently upgraded to look pedals and I thought since I was in marathon shape that I could hang with the big dogs. Wrong!!! I got dropped before we even headed away from Camp Bullis of course this was back in the day before 9/11 and the bases in and around SA extending their protection zones.
The first of this type was the Spectrum Club Ride http://www.spectrumsanantonio.com,
Next in the hierarchy is Bike City Chain Reaction http://www.bikecitychainreaction.com.
Next are the San Antonio Wheelmen http://www.sawheelmen.com, where normally the group going the furthest is the fastest.
Britton's Bicycle Shop http://www.brittonbikes.com
Cycle Logic rounds out the top as one of fastest http://www.cycle-logic.net.
The Hill Country Touring Group
Some local churches have beginner rides offered, Community Bible Church http://www.communitybible.com off 1604
Please feel free to add your comments as to why one or more of these should be listed differently. YMMV.