Today they are rolling out an update to address this. Here is the official statement:
“343 has announced that they are still working on the matchmaking issues but are indeed making progress on the situation. They rolled out an update earlier today, and another one is coming this weekend.
Tomorrow morning (Wednesday morning Pacific Time) we plan to roll out a number of server updates to help improve matchmaking. Again, these are server-side fixes, so you won’t need to take a content update or reboot your console; you should see the effects right away.
In addition, we are targeting another release for this weekend, which will address a number of other community priorities. An unrelated issue with services is also causing achievements to not unlock correctly, and we realize this just makes it all feel a bit worse.
A game as large as Halo: The Master Chief Collection has a massive surface area, and while we made every effort to have the best launch possible for our fans, issues surfaced with launch and we’re committed to improving this as fast as possible and get you all into matches. We’re committed to improving things as fast as possible so you can have the Halo experience you’ve been waiting for.”
You can expect this update anytime now if you have not already received it.