45 RPM
The slower a record spins, the more it's sound quality is affected. In order to deliver the best possible sound, the record must spin faster (45 RPM). However, when a record spins faster, the amount of information it can hold is reduced. The desire to improve sound quality, while ensuring adequate playtime, is a problem that still persists today. Obviously, the "convenience" drawback isn't an issue when you're willing to pay more for two 12-inch records instead of just one.