NEW YORK (TheStreet) -- The latest report on U.S. economic growth shows a continuation of not only the worst economic recovery since World War II, but also one of the worst six-year periods of economic growth since 1935. You read that right.

If the economy grows at 2.5% for all of 2015, it will rank as the 12th worst six-year period of real U.S. GDP growth out of 81 total observations since 1935.

The government said Thursday morning that the economy expanded at a 2.3% annual rate in the second quarter.

That was less than the 2.7% average forecast from economists.

Lurking behind that sluggish headline number was a worrisome detail: Gross private domestic investment (capital spending) contributed only 0.6 percentage points to the quarter's GDP (see chart below). That's its third smallest reading in the last 14 quarters.

And that's important, because sustained economic growth is a function of private-sector investment. 

True, first-quarter GDP growth was revised upward Thursday to 0.6% from an earlier reading of -0.2%, but 2015 remains on pace to mark the 15th straight year without an annual print of at least 4% real growth, a period during which compounded growth has been 1.8% (since the 2001 recession).

Yes, there was another pretty big recession during that period, but the U.S. economy is now six full years past the official end of that recession.

Table 1 Contributions to US GDP Growth

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Slower structural GDP growth has implications for securities valuations, both for equities and debt. Does it make sense for the cyclically adjusted price-to-earnings ratio of the S&P 500 to be at its fourth-highest level in history (trailing 1929, 2000 and 2007) if 1) corporate profit margins are almost 80% above long-term averages, and 2) long-term economic growth is going to be significantly slower than the pace supporting historical P/E ratios?

Of course not, and unless growth returns to a trajectory not seen since the 1990s, normalized equity earnings multiples will be ratcheted down to compensate investors via higher dividend yields for the lower long-term growth expectations.

Moreover, slower-than-expected GDP growth will result in greater deficits, which are already projected to explode in the U.S. and most developed countries.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.