You tell us that timekeepers have added a leap second to our clocks to match the Earth's spin, saying we are rotating "a bit slowly." The phrase should have been that we are rotating a bit slower and will continue to do so.
The moon, which has a tremendous gravitational effect on our planet, is moving away at the rate of nearly three inches per year, and as it does so, Earth's spin slows.
The moon, which is now 240,000 miles away, was created when the Earth was young, and it loomed as a giant in our skies, being only a few thousand miles away. A complete rotation of our orb took only six hours.