For Restclient version < 107:

should I use singleton pattern for RestClient or should I new it
everytime, if I new it everytime will any performance concern?

Recommended way to use RestSharp is to create a new instance per request.

It differs from Singleton approach recommended for HttpClient. And the reason is that under the hood RestSharp uses HttpWebRequest for HTTP interaction, not HttpClient. That’s why the usage model differs.

If I create it everytime do I get performance issue just like the
HttpClient?

The main reason why you shouldn’t create a new instance of HttpClient for each request is not a performance consideration. The time spent for creation and initialization will take a tiny fraction of time spent for following network call. The main reason to use singleton instance of HttpClient is the following:

HttpClient is intended to be instantiated once and re-used throughout
the life of an application. Instantiating an HttpClient class for
every request will exhaust the number of sockets available under heavy
loads. This will result in SocketException errors.

RestSharp does not use connection pool as HttpClient and does not leave opened sockets after the use. That’s why it is safe (and recommended) to create a new instance of RestClient per request.

Will you gain any performance improvement if you use reuse instance of RestClient? Well, you will save the time for creation of object and its initialization. However this time is very close to 0 and moreover it’s a tiny fraction of time spent for following network call. You don’t reuse other .NET objects like List<T> because of performance considerations, are you? You should do the same for RestClient. It’s just developed in a way that implies such usage scenario.