https://phys.org/news/2018-06-unhurried-hurricanes-tropical-cyclones.html

In the Atlantic Ocean basin, the slowdown was not dramatic, just 6 percent. But when Atlantic storms hit land—like Harvey—the study said the slowdown is a significant 20 percent. Atlantic storms that make landfall moved 2.9 mph (4.7 kph) slower than 60 some years ago, it said.

"The slower a storm goes, the more rain it's going to dump in any particular area," said study author James Kossin, a government climate scientist. "Hurricane Harvey last year was a great example of what a slow storm can do."