Browning et al (2006) found no difference in preferred walking speed between men and women of normal and obese weights. All subjects preferred a walking speed of around 1.42m/s (about 3 mph), which was determined to be the speed that maximized distance for effort. This is in contrast to previous results wherein Spyropoulous et al (1991) reported a preferred walking speed of 1.09m/s for obese men while Ohrström et al (2001) found a preferred speed of 0.75m/s for obese women.
Oberg et al's (1993) reported that "significant sex differences exist in all gait parameters" in their study of normal men and women aged 10-79. Women reached their fastest average walking speed (1.29m/s) in the 30-39 age group, whereas men topped out in the 15-19 age group, at 1.35m/s.
My coworker, the Evil Ganome, is fond of ranting about the slow walking speed of people blocking the sidewalks in front of him, which he attributes to their flip-flops. I tend to be offended by flip-flops for more aesthetic reasons, but agree that this particular choice in footwear will slow you down. I note this because I tended to believe that the stereotype of women walking slowly was based on the sadistic nature of women's shoe design, but this does not appear to be entirely the case. One thing that is not reported in any of today's studies is whether the differences in walking speed are proportional to differences in length of stride, although Oberg does note that women's step frequency is higher than men's, which suggests (to me) that women are used to trying to keep up with people with longer strides.