Mobile retail sites on average took more than 18 seconds to load Monday, twice their normal load time, the Mobile Commerce Index from Keynote, a mobile and website monitoring company, reported.
The same websites' traditional pages, the ones that load on laptop and desktop PCs, were loading in just a seventh of that time, CNN reported.
Tablets and smartphones are becoming popular with shoppers. The number of people using them to make online purchases rose by 70 percent over 2011.
Slow mobile website speeds encountered by frustrated shoppers puts "significant revenue at risk," Keynote said.
"The amount of holiday shopping now taking place on smartphones has simply grown too large for retailers to ignore," Aaron Rudger, mobile and web performance manager at Keynote, said.
Online shoppers typically won't wait more than 3 seconds for a website to load, Rudger said, and online retailers risk losing shoppers to competitors' sites if they don't present pages fast enough.