To reproduce:
Add a RadGridView with a GridViewComboBoxColumn and add 70000 items as datasource. You will notice that openning it is slow.

Workaround:

Use the following custom editor:

public class MyEditor : RadDropDownListEditor
{
    protected override RadElement CreateEditorElement()
    {
        return new MyEditorElement()
        {
            SyncSelectionWithText = false
        };
    }
}

public class MyEditorElement : RadDropDownListEditorElement
{
    public override void NotifyOwner(PopupEditorNotificationData notificationData)
    {
        if (notificationData.notificationContext == PopupEditorNotificationData.Context.TextChanged && !this.SyncSelectionWithText)
        {
            return;
        }

        base.NotifyOwner(notificationData);
    }
}