If this was the GPS speed, it may not have been the speed through the water, so, depending on the current, the boat may have been going a little slower.
Where I used to live, on the South West coast of Scotland, it was pretty common to get 7kts indicated on the GPS when sailing my old 22ft boat North or South. About 2kts of this was the tide running up or down the North Channel, though, so the boat was only really doing about 5kts through the water.
Jeremy