Advance Auto Parts sped up its site load time by 0.36 seconds, clocking in at 1.51 seconds on average for the two weeks Sept. 12-25, compared with last month’s average speed of 1.87 seconds, according to digital performance analytics company Catchpoint Systems Inc.

Shaving off a third of a second bumped the seller of car parts and maintenance equipment up nine spots to No. 4 on Internet Retailer’s monthly desktop performance index provided by Catchpoint Systems, which ranks the top 100 online retailers in the Internet Retailer 2016 Top 500 Guide according to web page load times. Last month, Advance Auto Parts, No. 77 in the Internet Retailer 2016 Top 500 Guide, ranked No. 13 in the index.

The retailer can credit  the nearly half a megabyte it cut from its home-page weight to its speedier site. Home-page weight shrunk to 1.68 megabytes, compared with 2.15 megabytes in August, a Catchpoint spokesman says. Advance Auto Parts did not respond to a request for comment.

The Home Depot Inc. (No. 7 in the Top 500) was the other newcomer to the top 10 this month, sliding into  the No. 10 spot, up from No. 14 last month.  Here are the top 10 performers:

Apple Inc. (No. 2), Shop.com (No. 59) and Sweetwater Sound (No. 89) each had speedy desktop and mobile site home pages and appeared on both the desktop and mobile September indexes.

“These companies should be commended for consistent performance excellence across their digital properties,” a Catchpoint spokesman says.

Overall, desktop sites loaded slower with an index average of 3.33 seconds compared with September’s monthly mobile site index, which had an index average of 2.77 seconds. That difference does not  surprise to Catchpoint CEO Mehdi Daoudi.

“It’s not that their desktop sites are overly fat, but rather that their mobile sites are exceedingly slim,” Daoudi says. “The retailers know that mobile is an increasingly important part of the shopping experience, and they are leaving nothing to chance when it comes to making sure mobile sites are as slick and fast as they can possibly be.”

Catchpoint Systems monitors each website’s home page with measurements taken from Catchpoint Systems’ in-country or in-region monitoring nodes, at intervals of five minutes for two weeks each month. Backbone monitoring nodes are the locations of Catchpoint’s devices that are near data centers operated by the main Internet Service Providers that provide service to a city. The nodes simulate end-user contact with each website. Catchpoint monitors webpage load time, availability, hosts and items. They are defined as follows:

Webpage load time: The time it takes for enough page elements to load for a consumer to begin interacting with a page, such as searching, tapping or scrolling. From a consumer point of view, the time it takes for the progress bar or spinning wheel to stop.

Availability: The percentage of time during the test week that the site can be successfully reached by a consumer.

Host: Any domain that delivers data, content or services over the internet to the site.

Items or Requests: Webpage components, such as files or images that a page loads from internal and external hosts or domains. These can include PDFs, PNGs, JPEGs and GIFs.

The top 10 retailers of the top 100 retailers in the Internet Retailer Top 500 Guide are ranked based on their desktop webpage load times.

For the two weeks Sept. 12-25, the average desktop web page load time among the top 100 retailers was 3.33 seconds compared to last month’s average 3.20 seconds. The average total downloaded onto the desktop site home page was 2.67 megabytes compared with 2.65 megabytes the previous month. The websites were available 99.78% of the test period compared with 99.82% last month. The average number of hosts was 52.38 compared with 51.67 in August. And the average total number of items on the home page was 172.89 compared with 172.19 last month.