Old post, but I thought I'd chime in…..this could be a two-fold issue. First, many times major technical changes are accompanied by a short-term performance decrement. When the athlete's body adjusts to the modified movement patterns and learns to apply force efficiently under the new conditions, performance should increase greatly. In a completely different scenario, I also wanted to add that athletes who make moderate changes in their mechanics often complain about getting slower when in fact they are the same speed or faster. This typically happens to athletes who have 'turnover addiction' and like the feeling of spinning their wheels without really applying any force. As a result they get a false read that they are running faster just because their moveing their limbs faster and their stride frequency is greater when in fact they are slower because they aren't applying ground forces efficiently and as a result they aren't producing optimal vertical displacement which leads to a myriad of problems, one of which is exagerated turnover.