Windows Forms App gives "not responding" message when i try to hit REST service on a user action

I am trying to create a web forms application which fetches data from database on clicking a button on the form and then uses that data to hit REST service and finally store items which have OK response. But the service takes approx 3-4 sec to respond to each request and total hits are approx : 80,000 and the forms app starts "not responding" as soon as i start hitting the REST service. PFB code for the app .Any help would be really great.

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;
using System.Configuration;
using RestSharp;
using Newtonsoft.Json;

namespace Test_Data_Generator
{
    public partial class Form1 : Form
    {

        string BaseDevURL = string.Empty;
        string BaseSIURL = string.Empty;
        string _currentProductItemID = string.Empty;
        List<StringValue> ValidProductItemsIDs = new List<StringValue>();
        private IRestResponse _response;
        private ServiceActions _RESTservice;
        private Dictionary<string, string> _requestHeaders;
        private DataTable ProductItemIDs;
        BindingList<StringValue> blist = new BindingList<StringValue>();

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            _requestHeaders = new Dictionary<string, string>
            {
                {"ContentType", "application/json"},
                {"Accept", "application/json"}
            };
        }

        private void btnGetTestData_Click(object sender, EventArgs e)
        {
            //Populate the data table on the basis of radio button which is checked
            if (!(rbDev.Checked) && !(rbSI.Checked))
                System.Windows.Forms.MessageBox.Show("Please check a radio button for desired environment");

            else if (rbDev.Checked)
            {
                ProductItemIDs = DataBase.GetValues(rbDev.Text);
                BaseDevURL =  ConfigurationManager.AppSettings["Service DEV"];
            }

            else if (rbSI.Checked)
            {
                ProductItemIDs = DataBase.GetValues(rbSI.Text);
                BaseSIURL = ConfigurationManager.AppSettings["Service SI"];
            }

            try
            {
                dgvDBdata.DataSource = ProductItemIDs;
                lblDbValuesCount.Text = ProductItemIDs.Rows.Count.ToString() + " rows";
            }
            catch (Exception ex)
            {
                System.Windows.Forms.MessageBox.Show("Exception caught : " + ex.Message);
            }

        }

        private void btnGetDataFromRESTService_Click(object sender, EventArgs e)
        {
             dgvValidPIDs.DataSource = blist;
            //Populate the data table on the basis of radio button which is checked
            if (!(rbDev.Checked) && !(rbSI.Checked))
                System.Windows.Forms.MessageBox.Show("Please check a radio button for desired environment");

            else if (rbDev.Checked)
            {
                _PITSservice = new ServiceActions(BaseDevURL);
            }
            else if (rbSI.Checked)
            {
                _PITSservice = new ServiceActions(BaseSIURL);
            }

            try
            {
                //Submit Rest request for each ProductItem ID retrieved from DB
                for (int i = 1; i < 100; i++ )
                {
                    _currentProductItemID = ProductItemIDs.Rows[i].ToString();

                    _response = _RESTservice.SubmitRestRequest(
                     null,
                     "ProductItemSummaries?ProductItemId=" + _currentProductItemID,
                     "Get",
                     DataFormat.Json,
                     DataFormat.Json,
                     _requestHeaders);

                    if (_response.StatusCode.ToString().Contains("OK"))
                        blist.Add(new StringValue(_currentProductItemID));
                    dgvValidPIDs.Refresh();
                }

            }
            catch (Exception ex)
            {
                System.Windows.Forms.MessageBox.Show("Exception caught : " + ex.Message);
            }
        }


    }

    public class StringValue
    {
        string _value;

        public StringValue(string s)
        {
            _value = s;
        }
        public string Value { get { return _value; } set { _value = value; } }

    }

}