We were there January 12-16 this year.
The Builders Convention was slated to draw over 120,000 attendees. Many of them were staying at Disney resorts but the parks were not that crowded. They did close MK one night because the Builders had a private party.
The college cheerleading championships were at Wide World of Sports and also at MGM. Again, not crowded but it did impact the MGM hours and the Indidana Jones show was closed because they were cheering on that stage.
There were multiple other sports things (Tae Kwon Do, 3 on 3 soccer, some sort of adult baseball leagues) but the parks were fine. We saw ONE group of Brazilian teenagers at MGM but they didn't bother us. In fact we saw them on the Great Movie Ride first thing in the morning and didn't see them again until Fantasmic that night and they were WAAAAY across the stadium from us.
The biggest worry we really had was weather. It POURED rain on our third day and our fourth day was cold, misty, and yucky. Pack for every possible weather scenario.
We went home the day before MLK Monday so I don't know how bad the crowds got. With the weather being yucky it might not have gotten more crowded after we left.