Based on David's skills and experience, I'm estimating his hourly rate to be around $125. This is lower than the average rate for software engineers in San Francisco, which is around $150-$200 per hour, due to the lack of specific information about his level of experience and the companies he has worked for. However, his skills in optimizing performance and web development are in demand, and he has experience in start-ups, which can command lower rates. A more experienced software engineer with similar skills might charge $175-$225 per hour, but David's profile suggests he is earlier in his career.