Still on the subject of being fast and good, team structure and process affect this massively.
To put it simply, big is stupid and cumbersome while small is clever and nimble.
Any sort of team should only be as big as it needs to be and, to be honest, smaller than that.
Both the agency team and also the core client team you work with.
Massive agency teams tend to, not just inhibit work getting out at a decent clip, they also inhibit quality.
Massive client teams will stop work getting approved any time and bestow an agonising death of a thousand cuts on it.
At a basic level, it's hard to keep everyone happy. In an agency team, everyone wants to contribute and feel they're part of it. Which only leads to compromise, which is not a reliable factory for good ideas.
Sometimes though, you're stuck with a big team, and let's face it, with the complexity of modern projects, it can be unnavoidable. Which means the best approach is benign dictatorship. A tight knit group of decision makers and a greater pool of surrounding 'doers'.
On the client side, decision by committee never works, because if you ask someone what they think, they feel obliged to tell you. Now, I've got to admit, I find creative reviews really hard, because, to be honest, at first, I don't know what I think, so I'm sneaky and make everyone else feedback first. If it's just me, I'll be honest and say I need to go away and think, to avoid saying something stupid.
I'm not horribly bad at judging ideas, but I need time to process them, we all do.
So if you're presenting stuff for the first time, you should try and insist it's to a core client team, and allow them space to think.
Otherwise you've got a blizzard of half formed opinions, even worse, they are from Jill from R&D and Bill from the insight deparment.
They will quickly become gospel, because no one wants to look like they've changed their mind in front of loads of people. Also, it's much easier to point out deficiencies in stuff than just say you like it, because no one wants to commit in big groups.
If you can get some sort of buy-in,especially with some sort of constructive feedback from small-ish team, they will feel they own the project and, when it's time to share it and get buy in from other stakeholders, they'll try and sell it in, rather than conform to group think.
Smallness is also important thanks to quirks of human psychology. Millions of years ago, we evolved an instinct to resist change, to stop predators noticing us. We're hardwired to play it safe. That's the little vioce in your head that says's 'But what if?", "But what everyone else does is..". The practical, side of you that likes to stick to what it's used to. We all have this little, safe, boring little bastard whispering in the back of our minds, trying to kill innovation and originality - the lifeblood of ideas that will work, because 'being right' isn't enough, being distinctive is the most important objective. All those car ads that look exactly the same? All those researched to death campaigns that have been beaten down to the same, obvious consumer insight? That's the result of Mr Safe getting his way.
The more people involved, the harder it is to avoid a critical mass of people giving in to the sneaky little bugger. Innovation comes from small, tight knit teams who instill energy into things, not brakes.
At some point, of course, you need to anaylse your logic, but while it's easy to post rationalise and shape something great, it's next to impossible to inject an idea into something that's correct but utterly dull and precictable.
Finally, a word on brainstorms. In the 1940's Alex Osborn bestowed the biggest trojan horse of mediocrity (to quote Richard Huntington) in the history of creative agencies.
An artificial situation where any idea is good idea means that really crap ideas can get through. The phenomenon of social loafing gives crapness a good chance too, because in big groups, we naturally try LESS hard, because we hope someone else will pull the weight, and anyway, the group takes responsibilty for success, not the individual.
Then there is the detrimental effect of social conformity. This is also a problem in focus groups. The people with the biggest mouths get heard the most and, because we all get along by mirroring and copying each other, it quickly becomes group think. Brainstorms are really a way for despots to get everyone to think they thought of their idea (which is how I approach moderation I'm ashamed to admit).
So again, forget big groups, aim for tight knit teams of people that trust each other to say what they really think.
Hope this helps.