The S&P 500 is at an all-time high, but just four of the 13 companies worth at least $1 trillion have outperformed the index this year. There is much consternation about how the rally is being led by the megacaps. But many of those stocks are trailing the market on a percentage-return basis.

Micron and Advanced Micro Devices have led the way, surging nearly 275% and just over 200%, respectively, thanks to strong demand for artificial-intelligence chips. Nvidia, on the verge of hitting a $6 trillion market valuation, is up nearly 30%. And Apple, with a market cap of almost $4.9 trillion, has gained about 23%. The S&P 500 is up 14.4%.

Meta Platforms is not far behind the index, up 12.7% this year. The launch of its new Muse AI agent has given the stock a recent boost. Google owner Alphabet, Amazon.com and Microsoft are each up between 10% and 11%. Eli Lilly and Broadcom have posted gains in the high single digits. Berkshire Hathaway is barely breaking even for the year, and Tesla is down 15%.

"For a cohort that has supposedly led, the [year to date] performance of the individual members hasn't been very impressive," said analysts at Bespoke Investment Group in a report Tuesday.

Elon Musk's other company, SpaceX, is the remaining trillion-dollar stock. SpaceX has taken off lately, and is now 30%, above its offering price. So it is outperforming the S&P 500's 6% gain since its June initial public offering. But it lacks a comparable full-year return, since it went public midyear.

So what's next for the largest megacaps?

Even though several of them have trailed the broader market this year, many have gained steam in recent months. As a result, the Bespoke analysts warned that six of the 13 trillion-dollar stocks look overbought on a technical basis: Micron, AMD, Nvidia, Meta, Microsoft and Tesla. And SpaceX, they argue, is extremely overbought. The remaining six are rated neutral by the Bespoke analysts.

Bespoke dug deeper into two of the 13 stocks in its report. The analysts argued that there could be trouble ahead for Meta, which is on the verge of what's known as a golden cross. That's when a stock's short-term moving average, typically 50 trading days, rises above its longer-term 200-day moving average.

"While technicians consider golden crosses to be a bullish technical pattern, in practice they don't always turn out that way," the Bespoke analysts wrote, pointing out that Meta stock hit an intermediate-term peak following golden crosses in 2018 and 2025.

Bespoke added that Alphabet could be a buy because its 50-day moving average has trended lower recently, even as its 200-day moving average has steadily risen. Bespoke said that this pattern has occurred 11 times since Alphabet went public in 2004 that this pattern has occurred. "These prior periods tended to be 'soft landings,'" the Bespoke analysts wrote, adding that "the stock generally rallied over the course of the next year."

"In fact, of the 11 prior occurrences, the stock was only lower a year later once," Bespoke added.

Still, an overbought reading doesn't necessarily mean a stock is about to fall. "Strong stocks often keep getting stronger when a firm trend is in place," said Barron's senior technical analyst Doug Busch.

To that end, Busch said that even though SpaceX's recent rally seems a bit much, "overbought conditions can remain in place for some time." His target on the stock is $195, 30% above current levels.

Busch added that Nvidia has only recently begun to show strong momentum. It lagged behind the broader market for the first 41/2 months of the year and continues to trail many other semiconductor stocks. "Look for [Nvidia] to join the rally and the world's largest stock can pull technology on its back into year-end," Busch said.

Busch also thinks that Microsoft's stock is not overbought and should continue to pick up steam. And he added that Apple, which Bespoke rates as neutral on a technical basis, "looks ready to run" and could hit $400 by the first quarter of 2027. That's 20% above its current price.

Investors are once again flocking to momentum stocks, even though several have underperformed this year. The Invesco Top QQQ ETF, an actively managed fund that focuses on the largest companies in the Nasdaq-100, is up 9% over the past three months, compared with a 4% gain in the S&P 500. The Roundhill Magnificent Seven ETF has surged 11.6% since early July.

Brian Hartigan, global head of ETFs for Invesco, told Barron's that the Top QQQ fund lets investors "dial in more" to exposure to big tech stocks. Nvidia, Apple, Microsoft, Amazon, Alphabet and Tesla make up nearly a third of the portfolio.

So even though most of these tech giants aren't beating the S&P 500 for the full year, many of them have surged in recent weeks and now look stretched. The ballyhooed broadening of the market rally has hit the pause button. The challenge for investors is deciding how much further the giants can run.