Etsy Inc. and Net-A-Porter are some of the most-improved retailers in terms of desktop website speed for the August and September Internet Retailer Desktop Performance Index.

The index ranks the speed of Internet Retailer’s Top 100 retailers’ desktop 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 Desktop Index ranked mobile sites between Aug. 14-27 and the September Desktop Index ranked sites between Sept. 11-24.

In August, luxury retailer Net-A-Porter (part of Yoox Net-a-Porter Group, No. 76 in the Internet Retailer 2017 Top 500) jumped to No. 40 on the index, loading in 3.22 seconds compared with a load time of 5.04 seconds in July when it ranked No. 79 on the index. The retailer elevated its performance by trimming the number of hosts and items on its site, a Catchpoint spokesman says.

In September, craft goods marketplace Etsy (No. 22 in the Top 500) also made a significant jump on the index to No. 12 from No. 18 in August. Although Etsy only shaved a tenth of a second—it loaded on average in 2.18 seconds in September compared with 2.28 seconds in August—this was enough to boost it five spots.

“Etsy improved wire time, which is the time the browser took loading network requests, which has had a positive impact on the page load time,” a Catchpoint spokesman says. Retailers can improve wire time by using a content delivery network or reducing the number of HTTP browser requests.

On the slower side of the top-performers index, Advanced Auto Parts (No. 84 in the Top 500) and Amazon.com Inc. (No. 1 in the Top 500)  each slipped on the index. Neither retailer responded a request for comment.

The automotive replacement parts retailer changed its page structure, which caused the site to slow down, the Catchpoint spokesman says. Advanced Auto Parts landed at No. 14 in August, loading in 2.16 seconds, compared with 1.81 seconds and the No. 7 spot in July.

Amazon increased the number of items on its page, its number of hosts and connections. These changes resulted in Amazon’s home page loading in 4.44 seconds, on average, between Sept. 11-24 compared with 2.66 seconds on the August Index. This caused Amazon to drop to No. 64 on the September Desktop Performance Index from No. 26.

For the two weeks of Sept. 11-24, the Top 100 retailers’ average load time was 3.95 seconds compared with 3.85 seconds the previous month, Aug. 14-27. The 100 leading North American retail sites are those ranked by Internet Retailer by online sales in 2016, according to Internet Retailer’s Top500Guide.com.

The average page weight during the two-week period was 2.99 megabytes compared with 2.97 megabytes the previous month, and the average number of hosts was 48.6 compared with 49.0. The average number of items per page was 178 compared with 175 the previous month. The 100 sites were, on average, 99.92% available Sept. 11-24, compared with 99.91% between Aug. 14-27.

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:

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 period 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.