In the western north Pacific, the slowdown's been 30 percent; in the "Australian" region it's 19 percent.
As storms move slower, they can unload more heavy rain and pound coastal areas longer, increasing damage potential.
He said Hurricane Harvey in Texas a year ago was a dramatic example of the consequences of a slow-moving or "stalled" tropical cyclone. But he says there's good evidence that the warming planet could weaken the global winds that push storms around.
For instance, it is expected that hurricanes will rain about 7 to 10 percent more per degree Celsius (1.8 degrees Fahrenheit) of warming, as the atmosphere retains more water vapor, Kossin explained.
"Long-duration or slower-moving storms, even when weaker, can have exacerbated impacts through prolonged wind exposure [in addition to] flooding", according to Colin Zarzycki, a project scientist with the National Center for Atmospheric Research who was not involved with the study.
When Hurricane Harvey devastated Houston in 2016, many scientists pointed to the fact that the storm hovered over the area for four days, dropping record amounts of water.
"I went in with that hypothesis and looked at the data, and out popped the signal that was much bigger than anything I was expecting", Kossin said.
While the new research suggests hurricanes and typhoons are slowing down over time, more work needs to be done to improve prediction models for how hurricanes may behave in the future. Atlantic storms that make landfall moved 2.9 miles per hour (4.7 kph) slower than 60 some years ago, it said.
Zarzycki's second point is that our means of studying hurricanes have also changed.
That means that storms farther from land in the earlier part of the study may not have had their speeds included in the study.
"These trends are nearly certainly increasing local rainfall totals and freshwater flooding, which is associated with very high mortality risk", he said.