c# - Loading data to dataGridView freezes the window during loading, why threading does not work here? - Stack Overflow

I have the following method that loads the data to DataGridView in a separate function. When I call the method in a button click, first two methods runs without hanging the windows since they dod not have any UI updates.

    private async void btnSearch_Click(object sender, EventArgs e)
    {

        btnFolder.Enabled = false;
        btnSearch.Enabled = false;
        btnStop.Enabled = true;

        try
        {
            var foldersTask = Task.Run(() => GetAccessibleDirectories(txtFolderPath.Text, chkSubFolders.Checked));
            var folders = await foldersTask;
            var BF = await Task.Run(() => getFolderdetails(folders));

            await Task.Run(() => this.BeginInvoke(new Action(() => loadFoldersToGrid(BF)))); // Runs on UI thread
        }
        finally
        {
            btnFolder.Enabled = true;
            btnSearch.Enabled = true;
            btnStop.Enabled = false;
        }
    }

But when it starts running 'loadFoldersToGrid,' the window freezes (it backs to normal once it is loaded completely.). I don't understand it why.

private async void loadFoldersToGrid(BrowserFolders BF)
{
    int i = 0;
    long MBcon = 1048576;
    long MB, Total = 0;

    dgFolders.Rows.Clear();
    dgFolders.Rows.Add(BF.folders.Count);

    foreach (var rec in BF.folders)
    {
        dgFolders.Rows[i].Cells[0].Value = i;
        dgFolders.Rows[i].Cells[1].Value = rec.folderPath;
        dgFolders.Rows[i].Cells[2].Value = rec.fileCount;
        long folderSize = GetFolderSize(rec.folderPath);
        string megabytes = (folderSize / MBcon).ToString("F1");
        dgFolders.Rows[i].Cells[3].Value = megabytes;
        dgFolders.Rows[i].Cells[4].Value = rec.fileCreated.ToString("MM/dd/yyyy");
        Total = Total + folderSize;
        i += 1;
    }

    txtFolderCount.Text = BF.folders.Count.ToString();
    txtSize.Text = string.Format("{0:N0} MB", Total / MBcon);
}

Any help ? tasks to not helping here.