5 minutes isn't actually that long, 10-30 minutes can be somewhat common currently depending on your preferred roles.
Silver 5 people have MMR's in the whole bronze 4 to actual silver 5 range since getting demoted tiers is quite hard to do, so it's not a huge surprise that you might have fallen if your MMR going into placements was actually in the bronze range potentially, although many people are placing lower then they 'are' but since they have an MMR still appropriate to them, then tend to get 25+ LP per win and can quickly climb to an appropriate division, so once you play a few more games to see what your LP gains/losses are like you'll be able to see where you stand.
(Also, I'm pretty sure bronze 5 probably has more players then any other league, simply because it has all the people 1 division's skill worse than bronze 4, but also everyone even worse than that since there's no where lower for them to be demoted to.)