WASHINGTON — The U.S. economy grew at an even faster clip in the spring than previously estimated. But there is concern that growth slowed in the summer, held back by global headwinds and turbulent financial markets. The Commerce Department says the overall economy expanded at an annual rate of 3.9 per cent in the April-June quarter, up from a previous estimate of 3.7 per cent. The new-found strength came from additional gains in consumer spending, business investment and residential construction. The second quarter gain in the gross domestic product, the economy's total output of goods and services, was a marked improvement from an anemic 0.6 per cent increase in the first quarter when the economy was buffeted by a harsh winter. Economists believe growth in the third quarter has slowed to around 2.5 per cent.