Apple just reported that it sold more than 5 million iPhones over the iPhone 5's opening weekend.

This is a very disappointing number.

It's below top Apple analyst Gene Munster's estimate of 6 million to 10 million. Worse, it indicates that growth may be slowing at Apple.

Take a look at this chart.

Opening weekend sales grew 70 percent between the 3GS and the iPhone 4, and then an astounding 135 percent between the 4 and the 4s. That's acceleration. iPhone 5 sales, meanwhile, grew only 25 percent. That's massive deceleration.

Digging deeper into the numbers, we find more bad news. Check out this second chart.

Apple sold the iPhone 5 in nine countries over its opening weekend. It sold the iPhone 4S in seven. It actually sold fewer iPhones per country this year than the last. That's not just deceleration, that's shrinkage:

Decelerating growth is not good for a company like Apple, which despite a modest P/E ratio, has one of the most generous trailing 12 month revenue multiples of any hardware company on the public markets.