I am on Ragnarok, which is a Legacy server, and the population on that is quite high.

There have been some occasions, usually around patch updates, where there has been a small wait queue to get onto the server. Generally though, there is always people running around. Also, as the Duty Finder matches with other servers across a global server (in my instance, the Chaos server), you have a greater pool of players when it comes to Duty Finder.

The only time I've ever experienced queue issues for Dungeons, Trials or Raids, is when I have queued as a DPS. If you queue as a Healer or a Tank, or party with one of these, usually you'll get straight in.