Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<PasswordHasherOptions>(options => options.IterationCount = 100_000);
    // Note: the number 100,000 is not a formal recommendation.  Test the performance of your login page before changing this.
    // Do what makes sense for your application, the hardware it runs on, and the hardware climate at the time when you read this.
    // Other entries remove for brevity.
}