Dell Inc. overtook Apple Inc. for the top spot on the December desktop performance index.

The December desktop performance index ranks the 10 fastest retail sites out of the top 100 retailers in the Internet Retailer 2016 Top 500 Guide for the two-week period Dec. 5-18, according to digital performance analytics company Catchpoint Systems Inc.

Dell (No. 3 in the Top 500) consistently ranks in the top 10 fastest sites, and rose one spot this month to clinch the No. 1 position. Other than a few e-retailers shifting around, such as Staples Inc. (No. 5 in the Top 500) moving to No. 3 from No. 5 last month,  the top 15 websites were almost identical between December and November, Catchpoint says.

This is likely because of online retailer’s freezing their site code, says Dritan Suljoti, co-founder of Catchpoint Systems. Retailers do not want to take any risks that could potentially jeopardize conversion during the busiest time of year, he says.

“With such huge traffic increases during the holidays, e-commerce IT operations teams go through months of testing to ensure that the website content and features will perform well for customers when the time comes,” Suljoti says. “To ensure this, almost all retailers implement these code freezes before November, so as to avoid making any major changes during the peak traffic months of November and December.”

Similarly, the overall performance of the retail sites of the top 100 retailers in the Top 500 in December only varied slightly compared with November, according to Catchpoint Systems. The average webpage load time for the 100 retailers in the two week period is 3.62 seconds, which is a marginal improvement from November’s two-week testing period at 3.66 seconds.

For the two weeks of Dec. 5-18, the average page weight was 2.53 megabytes compared with 2.60 megabytes in November, and the average number of hosts was 51 compared with 52 the previous month. The average number of items was 173, just slightly less than 174 in November. The top 100 sites were, on average, 99.87% available for the first two weeks in December compared with 99.52% available in November.

Internet Retailer and Catchpoint Systems produce two monthly site performance indexes, one for mobile sites and one for desktop sites.

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 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.