It's an inescapable truth in both life and the markets: Once you hit the top, there's no place left to go but down.

Such is the case with big-box retailer

Wal-Mart Stores

(WMT) - Get Report

, which was downgraded by a Goldman Sachs analyst on Monday.

Adrianne Shapira wrote in a note that there is limited opportunity for the company to generate the same rates of improvement as it has seen in prior quarters. She therefore lowered her rating to neutral from buy.

The news sent shares sliding by 3% to $48.41 in afternoon trading.

Over the past 18-months, Wal-Mart has actually benefited from consumers trading down to its value prices, more so than competitors like


(TGT) - Get Report

. But Shapira said, "Return on investment appears to be plateauing, which could stymie stock outperformance in the near term."

Shapira also expects investors to move into retailers who are "less staple-like names to take advantage of moderating declines in discretionary sales."

May marked the first month Wal-Mart refrained from reporting same-store sales numbers. In April, the company posted a 5% jump in comparable sales.

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