The home improvement chain also said the malware responsible for the breach has been removed from all stores.
The retailer topped both Internet Explorer and Firefox consistency tests in May.
OfficeMax.com had the best consistency score in Internet Explorer testing among large retailers sampled in May, says Compuware Gomez. Last month, the retailer had the most consistent Internet Explorer site loading time with a variation of 0.68 seconds. OfficeMax.com also had the best Firefox consistency score at 0.55 seconds.
Newegg.com (0.92 seconds) had the second-best consistency score in Internet Explorer tests, followed by Overstock.com and Dell.com (tied at 0.93), and Costco.com (1.03).
In Firefox testing, Apple.com followed OfficeMax.com with a consistency score of 0.81 seconds. Overstock.com was next (1.09), followed by OfficeDepot.com (1.25) and Dell.com (1.27).
The average Internet Explorer consistency score among the 22 retailers measured in May was 1.48 seconds. The same group of retailers produced an average Firefox score of 1.83 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.