If you've noticed that your queue times are higher in Rainbow Six: Siege, then you're not alone, especially if you're ranked higher than most players.
Thanks to a new process called clamping, the Rainbow Six team at Ubisoft is trying to find the sweet spot between giving high skill player instant queue times, and giving everyone balanced matches. The reasoning for this is that Ubisoft wants to eliminate matches where Diamond players are paired up against newcomers in the game - something they want to avoid even in Casual matches.
Another related issue that Ubisoft is working on is in the adoption of your ranked MMR into Casual matchmaking - this change was made without much fanfare from Ubisoft, and caused some of the longer queue times.
Ubisoft Community Developer Epi took to Reddit to talk about the change:
We wanted to do this to provide players with a more engaging experience in Casual. We know that it isn’t as much fun for our Diamond players to have to play Casual matches against other Diamonds as opposed to Silvers and Golds. We are comfortable with that, as it wasn’t very much fun for them to get stomped either. We would like to reiterate that this is a very temporary change. We have zero intention of this being a long term solution, and it is not our final design intent. To clarify, the method of matchmaking itself is what is temporary (MM based entirely on your Ranked MMR). The concept is not. We are working on ways to better tune it.
Some commenters on Reddit noted that using Ranked MMR to matchmake even in casual could cause some high rank players to purposely derank themselves just to have an easier time - but at the same time, waiting 45 minutes for a match also isn't acceptable either.
Ubisoft says that a fix for the latter issue will require maintenance and that they'll put out a notice for when it's on its way in Rainbow Six: Siege.