American Express may be facing its Microsoft moment.

ValueAct Capital, the investment firm run by Jeffrey W. Ubben, recently took a 1.1 percent stake in Amex, a $75 billion credit card and payments giant. With many of the usual tools used to fend off activist investors not obviously available at Amex, Mr. Ubben could use a strategy that worked for it at Microsoft. That leaves Amex’s chief executive, Kenneth I. Chenault, exposed.

Amex makes an atypical target for shareholder cage-rattling. The company usually reaches or exceeds its target of bolstering its earnings per share 12 to 15 percent annually. Last year’s 29 percent return on equity is standard fare, and far better than that of its closest rival, Discover Financial Services. The company’s capital is so high that the Federal Reserve allows the it to return more than 100 percent of earnings through dividends and buybacks. Investors seem to like management, not least Warren E. Buffett, whose Berkshire Hathaway is Amex’s largest shareholder, with 15 percent.

But most of its recent earnings per share performance has come from expense discipline and robust capital returns. Revenue growth has been more elusive. Mr. Chenault has failed in each of the last five years to hit his target of increasing annual revenue 8 percent. Two pure payment networks, Visa and MasterCard, usually enjoy double-digit growth.

Amex’s stock has underperformed, increasing 84 percent over the last five years, compared with 266 percent at Discover and 300 percent or more at Visa and MasterCard. A blend of the 12 public companies Amex cites as competitors grew 170 percent.