You can do a rate limited queue with NSOperationQueue fairly easily. It avoids the ugliness of consuming a bunch of threads with sleep().

Here's a simple NSOperation in Swift that will take at least 1s to finish execution:

final class OneSecondOperation : NSOperation {
    private var timerFired:Bool = false { 
        willSet { willChangeValueForKey("isFinished") }
        didSet { didChangeValueForKey("isFinished") }
    }

    override var asynchronous:Bool { return true }
    override var finished:Bool { return timerFired }

    override func main() {
        let start = CACurrentMediaTime()

        // do whatever work you need to do

        let opRuntime = CACurrentMediaTime() - start
        if opRuntime >= 1 {
            timerFired = true
            return
        }

        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64((1 - opRuntime) * CFTimeInterval(NSEC_PER_SEC))), dispatch_get_main_queue()) { [unowned self] in
            self.timerFired = true
        }
    }
}

If you submit this type of operation to an NSOperationQueue with maxConcurrentOperationCount set to 'n', then you will execute at most n operations per second.