The U.S. has fallen to No. 55 in LTE performance as speeds rise rapidly in countries that have leapfrogged some early adopters of the popular cellular system.

The average download speed on U.S. 4G networks inched up to 10Mbps (bits per second) in the June-August quarter, according to research company OpenSignal. That was an improvement from 9Mbps in the previous quarter, but the country's global ranking fell from 43rd as users in other countries enjoyed much larger gains. 

The U.S. was one of the first countries with commercial LTE service when Verizon Wireless launched its network in late 2010. But other countries that adopted the system later started with better technology, and some have secured more frequencies or rolled out enhancements that U.S. carriers haven't embraced as much, OpenSignal said. 

New Zealand scored the highest average speed in the quarter with 36Mbps, coming up from nowhere in the rankings. But perennial standouts like South Korea and Singapore kept getting faster, too. The average LTE speed in Korea is now 29Mbps (up by 4Mbps), and in Singapore it's 33Mbps, up by 5Mbps. 

OpenSignal collects data on cellular performance through a free app that mobile subscribers can use to measure the speed they're getting and find faster networks. The results announced Wednesday are based on readings from more than 300,000 users worldwide, the company said.

Countries like Hungary, the Dominican Republic and Morocco beat the U.S. in average LTE speed, but they aren't necessarily smartphone paradises. Mobile users in America get to use LTE more of the time, for example, because their carrier's networks are built out. Subscribers in the U.S. are on LTE 78 percent of the time, on average, making the country No. 10 for what OpenSignal calls "time coverage." Moroccan LTE may be fast, but 49 percent of the time, users there don't get it, for example.

The fact that LTE is nearly five years old in the U.S. may actually drag down its speed score, OpenSignal says. Consumers here have had years to buy the necessary phones and plans, so everyone from tech early adopters to cat-video fanatics are vying for every carrier's LTE capacity.

Still, some countries have opened up legitimately huge leads. The biggest standout is South Korea, where users enjoy LTE service 97 percent of the time. On carrier LG U+, it's 99.6 percent, so effectively subscribers never have to fall back onto 3G. Korea is also home to the fastest single LTE operator in the world, Olleh, with an average download speed of 30Mbps. SK Telecom and LG U+ are also near the top for speed.

They got there partly through new technology. All the LTE carriers in South Korea use Carrier Aggregation, part of the bundle of specifications called LTE-Advanced, to combine two or more spectrum bands into a single fat pipe. U.S. service providers are far behind on adopting Carrier Aggregation. 

Within the U.S., time coverage and speed varied widely among the major carriers, according to OpenSignal. Verizon was at 84 percent time coverage, followed closely by AT&T at 81 percent, but Sprint hit just 64 percent coverage. T-Mobile USA fell in between with 77 percent.

T-Mobile and Verizon tied for speed at 12Mbps, while AT&T averaged 8Mbps and Sprint came in at 6Mbps. None of them went up by more than 1Mbps between the second and third quarters.