How can i remove/delete digits in the beginning of a string ?

This is what i tried to do now this is the completed class code:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace ScrollLabelTest
{
    public partial class DisplayResponses : Form
    {        
        private List<string> nodesNames = new List<string>();
        private List<TreeNode> CurrentNodeMatches = new List<TreeNode>();

        public DisplayResponses()
        {
            InitializeComponent();

            addmore();
        }

        public void addmore()
        {
            foreach (List<string> l_branch in ListsExtractions.responsers)
            {
                TreeNode l_node = treeView1.Nodes.Add(l_branch[l_branch.Count - 1]);

                for (int l_count = 0; l_count < l_branch.Count - 1; l_count++)
                {
                    l_node.Nodes.Add(l_branch[l_count]);
                }
            }
        }

        private void DisplayResponses_Load(object sender, EventArgs e)
        {

        }

        private void treeView1_AfterSelect(object sender, TreeViewEventArgs e)
        {
            try
            {
                txtName.Text = "";
                txtParentName.Text = "";
                txtText.Text = "";
                txtTag.Text = "";

                if (treeView1.SelectedNode.Name != null)
                {
                    txtName.Text = treeView1.SelectedNode.Name.ToString();
                }
                if (treeView1.SelectedNode.Text != null)
                {
                    txtText.Text = treeView1.SelectedNode.Text.ToString();
                }
                if (treeView1.SelectedNode.Tag != null)
                {
                    txtTag.Text = treeView1.SelectedNode.Tag.ToString();
                }
                if (treeView1.SelectedNode.Parent != null)
                {
                    txtParentName.Text = treeView1.SelectedNode.Parent.Text.ToString();
                }
            }
            catch { }
        }

        private void treeView1_Click(object sender, EventArgs e)
        {
            ClearBackColor();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            ClearBackColor();
            try
            {
                TreeNode[] tn = treeView1.Nodes[0].Nodes.Find(txtNodeSearch.Text, true);
                for (int i = 0; i < tn.Length; i++)
                {
                    treeView1.SelectedNode = tn[i];
                    treeView1.SelectedNode.BackColor = Color.Yellow;
                }
            }
            catch { }
        }

        private void ClearBackColor()
        {
            TreeNodeCollection nodes = treeView1.Nodes;
            foreach (TreeNode n in nodes)
            {
                ClearRecursive(n);
            }
        }

        private void ClearRecursive(TreeNode treeNode)
        {
            foreach (TreeNode tn in treeNode.Nodes)
            {
                tn.BackColor = Color.White;
                ClearRecursive(tn);
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            ClearBackColor();
            FindByText();
        }

        private void FindByText()
        {
            TreeNodeCollection nodes = treeView1.Nodes;
            foreach (TreeNode n in nodes)
            {
                FindRecursive(n);
            }
        }

        private void FindRecursive(TreeNode treeNode)
        {
            foreach (TreeNode tn in treeNode.Nodes)
            {
                int index = tn.Text.IndexOf(" ");
                string text = tn.Text.Substring(0, index);
                tn.Text = tn.Text.Replace(text, "").TrimStart();
                if (tn.Text == this.txtNodeTextSearch.Text)
                    tn.BackColor = Color.Yellow;

                FindRecursive(tn);
            }
        }

        private void button3_Click(object sender, EventArgs e)
        {
            ClearBackColor();
            FindByTag();
        }

        private void FindByTag()
        {
            TreeNodeCollection nodes = treeView1.Nodes;
            foreach (TreeNode n in nodes)
            {
                FindRecursiveTag(n);
            }
        }

        private void FindRecursiveTag(TreeNode treeNode)
        {
            foreach (TreeNode tn in treeNode.Nodes)
            {
                if (tn.Tag.ToString() == this.txtTagSearch.Text)
                    tn.BackColor = Color.Yellow;

                FindRecursiveTag(tn);
            }
        } 
    }
}

I added this part in the method FindRecursive:

int index = tn.Text.IndexOf(" ");
string text = tn.Text.Substring(0, index);
tn.Text = tn.Text.Replace(text, "").TrimStart();

When i'm using a breakpoint i see it does removing the digits and point as i wanted.

If tn.Text was: 4. hello

So now it's: hello

The problem is that when i added this part of code everything got very slow taking forever.

If i remove this part of code with the IndexOf and Substring it's working fast but then i need to type in textBox2 also the digits: 4. hello

And not only hello as i wanted.