The coffee giant said comparable sales grew 1 percent over a year earlier in China. It\u2019s not exactly a roaring increase, but it\u2019s the same pace of growth that Starbucks recorded on this measure in its previous quarter, so it shows the company hasn\u2019t seen a sudden disturbance to its business there. And yet, investors shouldn\u2019t feel excessively comforted by Starbucks\u2019s latest showing in China, which the company has called its \u201csecond home market.\u201d It remains at the low end of what the Starbucks has promised over the long term, and is significantly slower than what it was even a year ago. Starbucks\u2019s performance in China isn\u2019t being shaped so much by political brinkmanship or economic headwinds; instead, it faces more prosaic challenges. First, Starbucks is feeling the heat of an insurgent competitor. Luckin Coffee, a Chinese startup, has been on a tear, as my colleague Nisha Gopalan has noted. It launched a little more than a year ago and quickly has gained traction, helped by its focus on speedy delivery and discounts. Luckin is valued at $2.2 billion, Bloomberg News has reported, and it plans to grow to an eye-popping 4,500 locations by the end of 2019. For context, Starbucks has been in China since 1999, and as of December, it had about 3,700 stores there. Starbucks is playing defense, including entering a partnership with Alibaba Group Holding Ltd. that allows it to offer delivery. But Luckin has come on extraordinarily fast and strong, and Starbucks is seeing the effects. That\u2019s not to say Starbucks is moving slowly with its own expansion these days. The company has said it is opening a store about once every 15 hours in China as part of a push to create a portfolio of 6,000 locations by 2022. The idea is to make sure it has early-mover advantage, blanketing the country with cafes even before a robust coffee-drinking culture has widely taken hold. There\u2019s logic to that strategy, but it\u2019s not without downsides. In fact, it is likely weighing on the company\u2019s comparable sales growth in the near term as new locations cannibalize business from old ones. Starbucks executives have said the bulk of its long-term sales growth in China is going to come from opening new stores, not from comparable sales growth. But I still can see why it might make investors skittish that older stores aren\u2019t building more of a following. That\u2019s especially true when you consider the challenges Starbucks faces in penetrating markets beyond top-tier cities such as Shanghai. As Sara Senatore, a restaurant-industry analyst at Bernstein has noted, Starbucks\u2019s relatively high prices make it more difficult for the chain to make inroads in metropolitan areas where incomes aren\u2019t as high. This challenge could be exacerbated if China\u2019s economy darkens. Starbucks can ill afford misfires in China. While its U.S. comparable sales exceeded analysts\u2019 expectations this quarter, rising 4 percent from a year earlier, the growth was once again fueled entirely by higher average checks. Transactions, a proxy for traffic, were flat from a year earlier, suggesting it has work to do in its home market. If it could rev into higher gear in China, it might be easier to overlook those signs of saturation in the U.S.