HONG KONG — It is one of the best-guarded borders in the world, and one of the most time-consuming to cross. Yet in the past few months, a new agreement has let people speed over it billions of times.

The border is the digital one that divides China from the rest of the world. It is laden with inefficiencies and a series of filters known as the Great Firewall, which slows Internet traffic to a crawl as it travels into and out of China.

Now, a partnership between an American start-up and a Chinese Internet behemoth has created a sort of fast lane to speed traffic across the border. In the process, the two companies are establishing a novel business model with implications for other American technology firms looking to do business in China’s politically sensitive tech industry.

The partnership, signed in July 2014, is between CloudFlare, a security company based in San Francisco, and Baidu, China’s equivalent of Google. Using a mixture of CloudFlare’s web traffic technology and Baidu’s network of data centers in China, the two created a service that enables websites to load more quickly across China’s border. The service, called Yunjiasu, began operating in December. It has a unified network that makes foreign sites more easily accessible in China, and allows Chinese sites to run in destinations outside the country.