Multiple people queuing up together doesn't slow down matchmaking. It actually speeds it up quite a bit. This is because it forces people to pick at least 3 roles for a duo or up to 5 roles for a 4 man, forcing people to take the off roles and 5 mans mean that it only has to create one team. Dynamic queue, I hate to say, actually speeds up matchmaking.
The role selection in general, however, slows it down too much for dynamic queue to fix. I'll try to explain it with completely made up numbers.
Let's say of every 10 people who queue up, 3 want mid, 2 want top, 2 want jungle, 2 want adc, and 1 wants support. Completely made up the numbers. Now what the system did before if find the first 10 players to queue up and put them into a game, but now since there's only one support in those first 10 players, it has to wait for the NEXT support to queue up which could take another 10 players, doubling queue times for the first 10. The 9 players from the second set now have to wait for TWO MORE SUPPORTS, which triples their queue time and it goes on and on. The only reason queue times aren't infinitely long is because dynamic queue forces 4 man premades to come with a support, along with 5 man premades, but it's still not enough to keep times under the 5 minute point from my experience and I'm only silver.
Halfway through this reply I was like "holy shit this is why that have to keep dynamic queue". There's an aha moment if I've ever had one but anyways if I was unclear or you have any questions ask away.