After running slow in July, Walgreen Co.’s mobile site was the fastest website in both August and September on the Internet Retailer Mobile Performance Index.

The index ranks the speed of Internet Retailer’s Top 100 retailers’ mobile home pages based on data provided by digital performance analytics company Catchpoint Systems Inc. Catchpoint provides Internet Retailer with two monthly site performance indexes: one for mobile sites and one for desktop sites. The August Mobile Index ranked mobile sites between July 31-Aug. 13 and the September Mobile Index ranked sites between Aug. 28-Sept. 10.

Walgreens.com loaded in less than 1 second on average on mobile devices both on the August (0.81 seconds) and September (0.84 seconds) indexes, according to Catchpoint data. This is particularly noteworthy as on the July Mobile Index, Walgreens.com loaded in 2.98 seconds, on average, and ranked 39th on the index.

The main reason for the retailer’s speedy site is because it greatly decreased the number of items on its site to 38 in August from 50 in July, and decreased the number of hosts on its site to 149 in August from 171 in July. Walgreens (No. 39 in the Internet Retailer 2017 Top 500) maintained these metrics in September, helping it keep its No. 1 position on the index for two months in a row. Walgreens declined to comment.

Microsoft Corp. (No. 95 in the Top 500) and Overstock.com Inc. (No. 30) also improved their rankings on the index by speeding up their sites. Microsoft.com ranks sixth in September with a load time of 1.56 seconds up from 15th in August when it loaded in 1.93 seconds.

Similarly, Overstock.com trimmed more than a half second off of its mobile site load time, loading on average in 3.26 seconds in September, bumping its rank up to 39 from 62 in August when it loaded in 3.96 seconds on average.

“Overstock.com has gradually reduced their page size and the website is constantly removing and adding hosts, images and scripts, which has resulted in the overall performance of the website improving over the last few weeks,” a Catchpoint spokesman says.

For the two weeks of Aug. 28-Sept. 10, the Top 100 retailers’ average load time was 3.63 seconds compared with 3.48 seconds the previous month between July 31-Aug. 1.  The average page weight was 2.33 megabytes compared with 2.25 megabytes the previous month, and the average number of hosts was 46.6, compared with 46.9. The average number of items was 157 compared with 154 a month ago.

The 100 sites averaged 99.92% availability Aug. 28-Sept. 10, the same as July 31-Aug. 13. These are the 100 leading North American retail sites by 2016 online sales, according to Internet Retailer’s Top500Guide.com.

Catchpoint monitors each website’s home page with measurements taken from Catchpoint’s 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.

Here is how each of those four metrics is defined:

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 two-week 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.