Yes, but iteration loop is faster only if folk is pre-prepared (it's not the first time they are thinking about the issues on the actual brainstorming b session).

Otherwise it's slower (bcs it takes everyone's time instead of just one person's).
You just spend minutes waiting for them to figure out why several suggestions are a non-starter for the most obvious reasons even to them.

For the 'iteration loop' to work properly you need a good starring point, a prototype of sorts (or several) for the group to workshop forward faster (conceptually speaking).

Not just timewise, but objective-wise too.
For most ppl such meetings are fairly taxing and time is limited so spending time on the meeting on something they could have easily figured out on their own hurts the success/quality rate.

But yes, each team/subject/industry/working environment is different, so my experience is just anecdotal data.

It could take a week for everyone so send in their 5 ideas

That's just them not willing to do it, a week of thinking would produce amazing results.