Here's a number that should make anyone with money in index funds sit up straight: The top 10 performers in the Nasdaq 100 over the past year are up an average of 784%, according to BTIG analyst Jonathan Krinsky. That's higher than the 622% average gain for the index's biggest winners in the year leading into the March 2000 peak.

Let me be clear about what this does and doesn't mean. It doesn't mean we're about to crash tomorrow. It doesn't mean you should sell everything and hide in cash. But it does mean that if you own a broad index fund and think you're playing it safe, you might want to look under the hood.

The current top 10 is dominated by one theme: AI infrastructure. SanDisk posted a nearly 4,000% surge. Western Digital, Seagate, Micron, Intel, AMD—these are the companies building the picks and shovels for the AI gold rush. Memory, storage, chips. The stuff that actually makes data centers run.

In 2000, it was a different story. Sure, some of today's winners were there too—SanDisk and Lam Research appear on both lists. Nvidia, Apple, and Adobe were dot-com darlings. But back then, the list was packed with companies that had no earnings, no business model, just a ".com" at the end of their name.

That's the good news. Today's winners are real companies with real products and—here's the kicker—actual earnings. Micron just posted a 682% earnings jump. These aren't vaporware startups burning venture capital.

But here's the bad news: concentration risk is off the charts. When you buy a Nasdaq index fund, you're not getting 100 equal bets. You're getting a handful of massive winners and a long tail of everything else. Krinsky's analysis shows the market is "hotter at the top, but more top-heavy underneath."

What that means in English: the biggest stocks are carrying the entire index. If those few names stumble, your "diversified" index fund isn't nearly as diversified as you think.

The median return for the top 10 today is actually lower than it was in 2000 (354% vs 455%). So while the average is higher, that's because a few extreme outliers—like SanDisk's 4,000% moonshot—are dragging the number up. In statistics, we call that skew. In investing, we call that fragility.

Now, does this mean the AI boom is a bubble? Not necessarily. The 2000 crash happened because companies were valued on "eyeballs" and "page views" instead of profits. Today's leaders are profitable, growing, and sitting on massive cash piles. The fundamentals are real.

But valuations are stretched. When stocks go up 784% in a year, they're pricing in a lot of perfection. Any stumble—a disappointing earnings report, a shift in AI spending, a regulatory crackdown—could trigger a fast unwind.

So what should you do? First, check your index fund holdings. If you're in the Nasdaq 100 or QQQ, you're heavily concentrated in these names whether you realize it or not. That's not inherently bad, but you should know what you own.

Second, don't panic. Selling just because something looks expensive is how you miss the best years. The top 10 in 2000 kept running for months after they looked "too hot." Timing the market is a fool's errand.

Third, if you're adding new money, maybe think twice about pouring everything into the same names that just went up 784%. Dollar-cost averaging into a diversified portfolio isn't sexy, but it's a hell of a lot better than buying the top of a parabolic move.

The 2000 comparison isn't a perfect analog. The fundamentals are stronger today. But human behavior—greed, FOMO, the belief that "this time is different"—that part never changes. And right now, the hottest corner of the market is running hotter than it did at the peak of the dot-com mania.

If that doesn't at least make you pause and reassess your portfolio, I don't know what will.