Starbucks shares tumbled nearly 16% following a disappointing fiscal second-quarter earnings report, with the stock down about 35% over the past year. The coffee giant faced challenges in its two largest markets, the U.S. and China, which together host over 60% of its global locations. In the U.S., store traffic declined and in China, aggressive promotions failed to sustainably boost customer visits amid macroeconomic pressures and intense competition.