That gain, however, has been largely powered by megacap tech stocks, according to data from Wisdom Tree. The so-called Magnificent Seven have surged 27%, compared with an 11% gain for the remaining stocks in the benchmark over the same period.

With Wall Street's surge to record highs, the U.S. stock market looks nearly as expensive as ever, and investors are debating whether the lofty valuations are a bearish signal or justified by the technology-heavy market's profit outlook.