Unable to load the service index from source

I want to download a System.Data.SqlClient in Visual Studio and then I download it from Manage NuGet Packages for Solution (right-click on the Solution Explorer). When I try to browse that, it looks very slow and I found another way to install the packages from the NuGet website using Package Manager Console by running this command,

NuGet\Install-Package System.Data.SqlClient -Version 4.8.5

but it looks very slow too. Is it another way to solve this problem? Thank you.

Edit:Try to access the URL (https://api.nuget.org/v3/index.json) it can't open, any suggestion will be appreciated.