In my opinion, and this is depending on the team, but fast paced is the only way to go.
I'm not, nor have I ever been, a fan of delayed rushes. Rushing a site is usually the best way to take it. Split rushes, one distract strats, are excellent. There's nothing I hate more than sitting in a staging area just waiting.
If you can practice at being fast paced, it's usually easy to overwhelm another team with your speed.
Well, more important than what pace you play at, in my humble opinion, is making your opponents play at your pace. If you want to win, you need to control the pace of the game.
Feel free to play slow or fast, but make sure your opponents are forced to play around you, rather than you playing around them. If you're playing slow, but your opponents are still playing their own fast pace, you're toast. However, if you can force them to play at your speed, they'll be out of their comfort zone, you'll be calling the shots, and will have a much easier time winning.
In regards to pace, though, I'm with Jake. I've never liked slow play. Although, I do love controlled play. Two totally different things in my book. Slow play is delaying, waiting without a purpose, trying to let the time psyche them out, etc. Controlled play is waiting in staging areas, etc, but with a distinct purpose. Such as, getting your picks before you rush. Rather than just rush blatantly in there, you try to get the person or people within the view of the AWP. If you get it, great, if you don't, you go ahead and rush. Controlled just seems to combine the best elements of slow and fast play together, achieving the ultimate in strategy.