A customer holds an iPhone 6 (R) and iPhone 6 Plus after the phones went on sale at the Fifth Avenue Apple store in Manhattan, New York September 19, 2014. Apple latest phone lured throngs of gadget lovers, entrepreneurs and early adapters to its stores in New York, San Francisco and other cities around the world in the latest sign of strong initial demand for the new, larger generation of iPhones. Reuters/Adrees Latif

While Apple’s latest iPhone 6 and iPhone 6 Plus have been received exceptionally well in the United States, the phones' initial adoption rates after the first weekend have been slower than that of the iPhone 5 in a comparable time period, according to a new report.

The initial usage growth for the 4.7-inch iPhone 6 has been impressive while the 5.5-inch iPhone 6 Plus is trailing its smaller counterpart. However, the early adoption rates of both devices are slower than that of the iPhone 5 because of supply delays, online ad network Chitika said in a report. The new iPhone models, which were released across the U.S., Canada and several other countries on Friday, garnered more than 4 million pre-orders in the first 24 hours after they became available, helping book nearly twice as many pre-orders as the iPhone 5 did in 2012.

To provide details about usage growth on the iPhone 6 and iPhone 6 Plus, Chitika examined millions of U.S. and Canadian iPhone-based online ad impressions that were generated between Sept. 19 and Sept. 21.

iPhone 6, iPhone 6 Plus usage rate. Chitika

According to the firm, iPhone 6 users surpassed a 1.5 percent usage share less than two days after the Sept. 19 launch of the device. That figure is nearly one percentage point behind what was observed for the iPhone 5 over the same time period following its public release. But, it is significantly more impressive than the Samsung Galaxy S5, which took more than a week to generate a 1.5 percent share of Samsung smartphone web traffic.

iPhone 6 adoption. Chitika

“The iPhone 6 Plus exhibiting a slower growth rate isn’t a tremendous surprise due to the phone’s size giving it more of a niche appeal as opposed to the 4.7-inch iPhone 6,” the Chitika report said. “Additionally, while supply delays have impacted the ability to meet demand for both devices, the iPhone 6 Plus has been selling out at retail stores much more frequently than the traditional model.”

Apple has not so far provided a country-specific breakdown of its record-setting iPhone 6 pre-orders. However, according to Chitika, it can be assumed that a considerable percentage of those purchases were made by consumers in the U.S. and Canada.

The early adoption rate for the iOS 8 has also been slower than that of its two predecessors -- iOS 7 and iOS 6. According to Chitika, iOS 8 was running on about 7.3 percent of iOS devices 24 hours after it was released. In comparison, iOS 7 was installed on 18.2 percent of devices after the first day of its availability while iOS 6 was running on 14.8 percent of devices within 24 hours of its release.