On 1/2/20, flight 2381, 420pm departure. The boarding pass indicated a boarding time of 350pm, so we ate and used the restroom. We purchased early boarding. When we got to the gate at 345pm, there was no line because everyone else had boarded the plane! Southwest needs to better inform passengers of early boarding. The flight itself didn't pull away from the gate until after 430pm, there was not enough air, so most of the passengers sat there in the warm stuffy air. My advice is to get early boarding and stand by the gate at least 45 minutes before boarding. This is the only time out of over a hundred flights on Southwest that they have ever boarded early. It must be due to the route.