If you don’t pay $99 each year for an Amazon Prime membership, which promises two-day shipping on all orders, regular shipping on Amazon purchases might be slower than it was last year, according to a study from customer service metrics company StellaService.

In an interview with Forbes, StellaService VP of Research Kevon Hills said the company tracks how quickly orders from 40 companies are fulfilled. Amazon fell outside of the top 10 fastest companies for the first time this year, losing ground to companies like BestBuy and Apple.

Hills said the speed of shipments to Prime customers is unchanged, indicating that Amazon is throwing considerable investment at serving its rapidly growing Prime base—which increased by 51 percent in 2015. Although Amazon does not reveal numbers about how many customers are Prime members, analysts have estimated that Prime membership totals at least 46 million people and could be as high as 80 million.

Still, the company has often struggled to show consistently strong profit margins, which has been attributed in part to the increasing cost of Prime deliveries. But Amazon’s emphasis on Prime seems to suggest that it’s working to serve its more lucrative customers first and letting service to non-prime customers slip.

Ars has contacted Amazon but has not yet received a response. We will update this post if we hear back.

Hills also told Forbes that other companies are getting faster at shipping products. “Three years ago, if you delivered a package in four days, that was worthy of a top 10 consideration. Now, if you deliver a package in four days, that puts you at 20. The industry is getting faster, and we’ve seen Amazon fall off a little bit.”