Going on a trip from the USA to China is very slow.
So when someone says the slow boat to China they're saying you or themselves are doing something very slow and it will take awhile to finish or accomplish the goals or whatever they're trying to do.
The saying slow boat to China originated from playing Poker where the winner of the game achieved the poker winnings at a slow and steady pace.
When you're taking a boat to China you'd get there at a slow and steady pace.