This expression was used in our home today in an attempt to have one of our grandchildren get ready for an outing a bit faster. Idiomation says “slower than molasses in January” is the most frequently used version and has been used to describe something or someone as “painfully slow” since somewhere between 1840 and 1872. However, there was a disaster involving molasses in January when it would have been better if it had moved slowly. A tank containing 2.5 million gallons of molasses exploded on January 15, 1919. The 30 foot tall tidal wave of molasses calculated to have been moving at 25-30 miles per hour “killed 21 people, crumpled a steel support for an elevated train, and knocked over a fire station.”
I was skeptical that the Great Molasses Flood might be an internet hoax until I found a site with multiple photos of the aftermath that look quite real.