Blockbuster.com, finished with the best composite score among large web retailers for mobile performance in September, says Compuware. Compuware tests 20 large retailers every month for mobile response time, availability and consistency on Apple’s iPhone for three mobile carriers: AT&T, Sprint and Verizon Wireless.

Last month, Blockbuster.com produced an average mobile response time of 3.98 seconds, an availability score of 99.44% and an average consistency score of 5.48 seconds, giving the retailer the best score on all three measures across all three carriers.

Sears.com finished second in the rankings with an average response time of 2.63 seconds, an availability score of 98.74% and an average consistency score of 3.81 seconds. BarnesandNoble.com placed third in the September rankings with an average response time of 4.75 seconds, an availability score of 99.33% and an average consistency score of 7.73 seconds. Williams-Sonoma.com and Rakuten.com rounded out the top 5.

Click here for the full mobile test results.