The beginning of your flight is the best time to use Internet
Like, as soon as you hear that magic “ding” that announces “time to get to work,” start downloading like you’re a 12-year-old with parents on their way home. Or, do whatever it is your doing that involves the Internet. As is the case with most connections, the more people online, the slower it gets. For the first 20 minutes of the flight, I've found in-flight Wi-Fi is almost usable.
Unless... you're flying over water
Trans-Atlantic/-Pacific flights are another deal but on flights across the Western Hemisphere, in-flight Wi-Fi is beamed up from towers on the ground, not down from a satellite over your head. So, if your plane is crossing a large body of water, don’t count on getting any service once offshore. (Flights over the Gulf of Mexico often stay close enough to land that this isn’t an issue.) Same goes for crossing borders: your Wi-Fi will work for a hundred miles or so once you cross into Mexico, but once that US signal goes out, so does your connection.