UI lags and is choppy when using Glide to load images in a RecyclerView

I have a RecyclerView that loads images from URLs using Glide. Now the URLs are retrieved from Firebase using pagination as you can see below. The issue is that when the MainActivity (which contains the below code and the recyclerview) is first initialized there is a substantial lag in the UI (very laggy and choppy scrolling, options menu takes 3 seconds to open etc.) and the images take a while to load. After i scroll down though and reach the end of the RecyclerView for the first page of data, the OnScrollListener is triggered and i start loading new data from a new query. I've tried my best to optimize what Glide does based on suggestions from a user on another post i made and i also set the adapter.setHasFixedSize to true without luck. Any idea what's happening here? Am i hanging the UI thread somehow despite the queries being async?

EDIT : Could Glide be causing the lag on the main Thread due to it having to load multiple images into the recycler view's imageViews? And if so, what can i do to counter that?

Here's how i handle the pagination of the data i get from Firebase and notify the adapter:

class MainActivity : AppCompatActivity() {

private val TAG: String = MainActivity::class.java.simpleName // Tag used for debugging
private var queryLimit : Long = 50 // how many documents should the query request from firebase
private lateinit var iconsRCV : RecyclerView // card icons recycler view
private lateinit var lastVisible:DocumentSnapshot

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    val rootRef: FirebaseFirestore = FirebaseFirestore.getInstance()
    val urlsRef : CollectionReference = rootRef.collection("CardIconUrls")
    val query : Query = urlsRef.orderBy("resID",Query.Direction.ASCENDING).limit(queryLimit) // create a query for the first queryLimit documents in the urlsRef collection

    // Setting Toolbar default settings
    val toolbar : Toolbar = findViewById(R.id.mainToolbar)
    setSupportActionBar(toolbar) // set the custom toolbar as the support action bar
    supportActionBar?.setDisplayShowTitleEnabled(false) // remove the default action bar title

    // RecyclerView initializations
    iconsRCV = findViewById(R.id.cardIconsRCV)
    iconsRCV.layoutManager = GridLayoutManager(this,5) // set the layout manager for the rcv
    val iconUrls : ArrayList<String> = ArrayList() // initialize the data with an empty array list
    val adapter = CardIconAdapter(this,iconUrls) // initialize the adapter for the recyclerview
    iconsRCV.adapter = adapter // set the adapter
    iconsRCV.setHasFixedSize(true)

    iconsRCV.addOnScrollListener(object:OnScrollListener(){
        override fun onScrollStateChanged(recyclerView: RecyclerView, newState: Int) {
            super.onScrollStateChanged(recyclerView, newState)

            if(!iconsRCV.canScrollVertically(1) && (newState == RecyclerView.SCROLL_STATE_IDLE) && ((iconsRCV.layoutManager as GridLayoutManager).findLastVisibleItemPosition() == (iconsRCV.layoutManager as GridLayoutManager).itemCount-1)) {
                Log.d(TAG,"End of rcv-Starting query")
                val nextQuery = urlsRef.orderBy("resID",Query.Direction.ASCENDING).startAfter(lastVisible).limit(queryLimit).get().addOnCompleteListener { task ->
                    if(task.isSuccessful) {
                        Log.d(TAG,"Next query called")
                        for(document:DocumentSnapshot in task.result!!) {
                            iconUrls.add(document.get("url").toString())
                        }
                        lastVisible = task.result!!.documents[task.result!!.size()-1]
                        adapter.notifyDataSetChanged()
                    }
                }
            }
        }
    })

    query.get().addOnCompleteListener {task: Task<QuerySnapshot> ->
        if(task.isSuccessful) {
            Log.d(TAG,"Success")
            for(document:DocumentSnapshot in task.result!!) {
                Log.d(TAG,"Task size = " + task.result!!.size())
                iconUrls.add(document.get("url").toString()) // add the url to the list
            }
            lastVisible = task.result!!.documents[task.result!!.size()-1]
            adapter.notifyDataSetChanged() // notify the adapter about the new data
        }
    }
}

Here's the recyclerview adapter:

public class CardIconAdapter extends RecyclerView.Adapter<CardIconAdapter.ViewHolder> {

    private List<String> urlsList;
    private Context context;

    class ViewHolder extends RecyclerView.ViewHolder {
        ImageView iconImg;
        ViewHolder(@NonNull View view) {
            super(view);
            iconImg = view.findViewById(R.id.cardIcon);
        }
    }

    public CardIconAdapter(Context cntxt, List<String> data) {
        context = cntxt;
        urlsList = data;
    }

    @NonNull
    @Override
    public CardIconAdapter.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view =  LayoutInflater.from(parent.getContext()).inflate(R.layout.card_icons_rcv_item,parent,false);
        return new ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull CardIconAdapter.ViewHolder holder, int position) {
        RequestOptions requestOptions = RequestOptions.diskCacheStrategyOf(DiskCacheStrategy.ALL);
        GlideApp.with(context).load(urlsList.get(position)).thumbnail(0.25f).centerCrop().dontTransform().apply(requestOptions).into(holder.iconImg);
    }

    @Override
    public int getItemCount() {
        return urlsList.size();
    }
}

I finally figured out the issue after a lot of trial and error. My first mistake was not posting my xml layout file for the recyclerview item because that was the source of the performance issues. The second mistake was that I was using a LinearLayout and had set its own layout_width and layout_height attributes to 75dp instead of the ImageView's which is nested inside of the LinearLayout and was using wrap_content for the ImageView's respective attributes. So to fix the performance issues i did the following :

1. I changed the LinearLayout to a ConstraintLayout (i read that it is much more optimized in general)
2. I set the ConstraintLayout's layout_width & layout_height attributes to wrap_content and finally
3. I set the actual ImageView's layout_width & layout_height attributes to 75dp which is the actual size that i want the image to be

Here's the final layout file for each item of the recyclerview after the changes :

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:orientation="horizontal"
    android:padding="5dp"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content">

    <ImageView
        android:id="@+id/cardIcon"
        android:layout_width="75dp"
        android:layout_height="75dp"
        android:contentDescription="cardIcon"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:srcCompat="@tools:sample/avatars" />
</androidx.constraintlayout.widget.ConstraintLayout>