How to get data instantly from a api?

I am using newtonsoft to get json data like
HttpClient client = new HttpClient();
var response = await client.GetStringAsync("http:................");
var items = JsonConvert.DeserializeObject<List>(response);

But it take too much time to show in a list view as there are only 3000++ data.
It take much time for searching also . how can i improve the search and getting data quickly ?