iOS custom keyboard app extension UICollectionView not scrolling smoothy

Question

I'm building a custom keyboard, I'm trying to recreate the iOS emoji keyboard. So I created a UICollectionView and each cell contains an emoji (UILabel).

But it's not scrolling smoothy. I don't know if it comes from the UICollectionView of something else.

• Emojis are in a plist file, I open it in viewDidLoad
• I'm using a xib cell file for my UICollectionViewCell (since there is no other way with a custom keyboard)
• I made a test by removing the UILabel from the UICollectionViewCell and it's still not scrolling smoothy

Any clue?

---

EDIT

var emojis=[[String]]()

override func awakeFromNib(){
    dataSource=self
    delegate=self

    let nib=UINib(nibName: "EmojiKeyCell", bundle: nil)
    register(nib, forCellWithReuseIdentifier: cellId)
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell=collectionView.dequeueReusableCell(withReuseIdentifier: cellId, for: indexPath) as! EmojiKeyCell
    let emoji=emojis[indexPath.section][indexPath.item]
    cell.emojiKey.text=emoji
    return cell
}