Top online merchants provide shoppers with highly functional e-commerce sites. However, many of those added functions are slowing down their web performances, finds a new report from research firm Retail Systems Research LLC.

RSR’s recently published “2018 E-commerce Website Performance: The Stakes are Increasing, but are Retailers Falling Behind?” evaluated 79 retailers between April and May 2018 that ranked between 50 and 150 on Internet Retailer’s 2018 Top 500. RSR scored each retailer based on web speed on both desktop and mobile, ease of shopping and which technology features the retailer provides, such as product reviews, recommendations, personalization and live chat.

The highest score a retailer could earn was a 77. Retailers’ average score was 34.7 compared with 31.2 on average a year earlier.

Adidas AG (No. 61 in the Internet Retailer 2018 Top 500), The Men’s Wearhouse Inc. (No. 109), Ulta Beauty (No. 91), 1-800 Contacts Inc. (No. 100) and Edible Arrangements (No. 123) were the highest scorers.

Adidas topped the list overall for its notable performance in the shopper experience category, in which it received 19 out of a possible 23 points. Adidas received high scores for its mobile navigation and for having subtle pop-ups that didn’t impede the overall shopping experience, says Bob Buffone, chief technology officer at web performance firm Yottaa, which sponsored the report.

Like Adidas, Ulta Beauty has a mobile site with a number of features that were similar to the desktop site that aim to provide shoppers with a consistent feel on both sites, Buffone says.

Adidas also is one of only three retailers that RSR examined using a progressive web app, or PWA, which is a form of mobile web design that has app-like features—such as swipeable carousel images—and is typically fast loading.

Overall, retailers are committed to making their sites beautiful, intuitive and shoppable, especially on mobile devices, RSR writes. For example both athletic wear retailer Lululemon Athletica Inc. (No. 90) and brand manufacturer Tommy Hilfiger had video content on their mobile homepage hero images, and Crate and Barrel (owned by Otto Group, No. 80) and Kay Jewelers (owned by Signet Jewelers, No. 102) have mobile sites with more intuitive navigation than their desktop site, according to the report. Plus, brand manufacturer Ralph Lauren Media LLC (No. 76) offers live chat on its mobile site, while keeping it out of the way halfway down on the side of each page.

“These little touches make a big difference and quickly add up to a mobile experience that feel far more modern than what’s currently offered on aging desktop sites,” RSR writes in the report.

However, the report finds that many retailers are not providing optimal web performance, with many e-commerce sites taking longer than three seconds to completely load and longer than 0.1 seconds for any content to appear on the site, which is part of the criteria RSR used to score retailers.

A reason for the slower performance, according to the report, is the large number of vendors on the site powering added features, such as social embeds or product reviews. Of the 79 retailers in the report, the average number of site features dependent on a third party was between 25 and 34, and the average number of third-party requests, or calls a vendor has to make to a server for the page to load, was 139, according to RSR data.

“Third-party technologies offer retailers best-in-class functionality as well as speed-to-market, which is critical in today’s fast-changing retail landscape,” Buffone says. However, he adds, these technologies impede load time.

“The page needs to load in at least three seconds or shoppers will start leaving the site,” Buffone says. “It’s a daunting challenge for retailers as they need the third parties to optimize shopper experience but also need fast performance.”

RSR suggests retailers shop their own site to determine pain points, and to assess which features are necessary. Plus, retailers can shop their competitor’s sites and compare the experiences.

“Retailers need to have a strategy and technology to manage these add-ons before they negatively affect performance any further.”