Transnational Internet performance is an important indication of a country's level of infrastructure investment, globalization, and openness. We conduct a large-scale measurement study of transnational Internet performance in and out of 29 countries and regions,and find six countries that have surprisingly low performance. Five of them are African countries and the last is mainland China, a significant outlier with major discrepancies between down stream and upstream performance. We then conduct a comprehensive investigation of the unusual transnational Internet performance of mainland China, which we refer to as the "Great Bottleneck of China". Our results show that this bottleneck is widespread, affecting 79% of the receiver-sender pairs we measured. More than 70%of the pairs suffer from extremely slow speed (less than 1 Mbps)for more than 5 hours every day. In most tests the bottleneck appeared to be located deep inside China, suggesting poor network infrastructure to handle transnational traffic. The phenomenon has far-reaching implications for Chinese users' browsing habits as well as for the ability of foreign Internet services to reach Chinese customers.