The online-only retailer grew sales by 11.8% in 2014.
The retailer topped both Internet Explorer and Firefox consistency tests last month.
Costco.com had the best consistency score for Internet Explorer testing among large retailers tested in November, says Compuware. Last month, the retailer had the most consistent Internet Explorer site loading time with a variation of only 0.48 seconds. Costco also had the best Firefox consistency score at 0.33 seconds. The lower the variation, the more consistently the site loads.
Apple.com (0.66 seconds) placed second in Internet Explorer tests. Dell.com (0.80) came in third, QVC.com finished in fourth (0.89) and Amway.com (0.98) came in fifth.
In Firefox testing, Apple.com came in second place behind Costco with a deviation of 0.43 seconds. Dell.com took third place (0.69), Amway.com finished fourth (0.80) and it was followed by Overstock.com (0.90).
The average Internet Explorer consistency score among the 23 retailers measured in November was 1.58 seconds. The same group of retailers produced an average Firefox score of 1.86 seconds.
The consistency ratings take into account differences in the speed of web page delivery across multiple visits.
Click here for the full test results.