为什么我在collectionView中的单元格的形成速度比接收数据的方法更快？

DispatchGroup().enter()
// 异步加载数据
DispatchGroup().leave()
DispatchGroup().notify(queue: .main) {
    self.collectionView.reloadData()
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! CustomCell
    
    DispatchQueue.global(qos: .userInitiated).async {
        // 复杂的数据处理逻辑
        let data = self.processData(indexPath.row)
        
        DispatchQueue.main.async {
            // 更新 UI
            cell.configure(with: data)
        }
    }
    
    return cell
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! CustomCell
    
    if self.data[indexPath.row] == nil {
        cell.configure(with: placeholderData)
    } else {
        cell.configure(with: self.data[indexPath.row]!)
    }
    
    return cell
}