W.W. Grainger Inc. and Groupon Inc. both improved the speed of their mobile websites last month, which helped them secure the top spots on the Internet Retailer Mobile Performance Index for the month of May.

The index monitored Internet Retailer’s Top 100 retailers’ mobile home pages from May 8-21, according to 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.

Grainger (No. 11 in the Top 500) loaded on average in 1.40 seconds in May compared with 1.67 seconds the previous month. The retailer jumped up seven spots in the index to the No. 2 spot after it cut down on the time spent loading networks requests, according to Catchpoint. The vendor attributes the load time reduction to a decrease in the number of hosts—which is any domain that delivers data, content or services over the internet to the site—on the mobile home page to nine hosts in May, down from 11 in April according to Catchpoint. This is significantly less than the index average, which is 46 hosts per mobile home page from May 8-21, according to Catchpoint data.

Groupon (No. 27) shaved 0.2 seconds off of its mobile site, which meant it loaded in 1.45 seconds, which was enough to bump up the retailer to No. 4 on the index from No. 7 the previous month. Groupon improved its speed by reducing its overall page size and the number of images on its home page, Catchpoint says.

The Home Depot Inc. (No. 8) also made the top 10 list this month, moving up seven spots to No. 10 from No. 17 in April. To make the site faster, the retailer cut down the number of hosts and items on its mobile home page. Home Depot also used a script loading technique called asynchronous loading, which ensures slow-loading scripts do not interrupt the loading of other site elements, Catchpoint says.

“The number of scripts on the page has also come down drastically which has had a positive impact on the page load time,” a Catchpoint spokesman says. “Keeping the number of scripts down, as well as loading them in the most efficient manner possible, including asynchronous loading, are relatively simple web performance optimization techniques. Asynchronous loading helps ensure scripts ‘play nicely’ with other website elements.”

For the two weeks of May 8-21, the Top 100 retailers’ average load time was 3.34 seconds compared with 3.31 seconds the previous month. The average page weight was 2.18 megabytes compared with 2.25 megabytes the previous month, and the average number of hosts was 46 compared with 45. The average number of items was 148 compared with 166 a month ago.

The 100 sites were, on average, 99.89% available for May 8-21 compared with 99.94% available April 10-23. These are the 100 leading North American retail sites by online sales for 2016, 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.

They are defined as follows:

Web page 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: Web page 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.