UITableView in Swift: Offscreen cells are not pre-loaded

I have a little problem with UITableView. Cells that are offscreen are not loaded when I first scroll down. When I scroll up (so that the not showing cells are offscreen again) and scroll down again they suddenly show up. I think this has something to do with dequeueReusableCellWithIdentifier. Any idea how to fix this?

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath) as! CustomCellClass
return cell