asp.net core 2.0 cookie authentication with session variable - asp.net-core-mvc-2.0

Question

I'm trying to implement asp.net core cookie-based authentication. So I have added the code below to my startup.cs
services.AddAuthentication(CookieAuthenticationDefaults.AuthenticationScheme)
.AddCookie(CookieAuthenticationDefaults.AuthenticationScheme, options =>
{
options.AccessDeniedPath = new PathString("/login");
options.LoginPath = new PathString("/login");
options.SlidingExpiration = true;
});
and I'm sign-in using the code below
[HttpPost]
[ValidateAntiForgeryToken]
[Route("login")]
public async Task<IActionResult> Login(AuthViewModel authView)
{
if (ModelState.IsValid)
{
var (status, message, SigninUser) = await authentication.Authenticate(new User()
{
email = authView.Email,
pwd = authView.Password
});
if (status)
{
List<Claim> claims = new List<Claim>
{
new Claim(ClaimTypes.Name, "App Member"),
new Claim(ClaimTypes.Email, SigninUser.email)
};
ClaimsIdentity identity = new ClaimsIdentity(claims, "cookie");
ClaimsPrincipal principal = new ClaimsPrincipal(identity);
await HttpContext.SignInAsync(
scheme: CookieAuthenticationDefaults.AuthenticationScheme,
principal: principal,
properties: new AuthenticationProperties
{
IsPersistent = authView.RememberMe,
ExpiresUtc = DateTime.UtcNow.AddYears(1)
});
HttpContext.Session.Set<User>("session_user", SigninUser);
if (Url.IsLocalUrl(authView.returnUrl))
return Redirect(authView.returnUrl);
else
return RedirectToAction("Index");
}
else
{
authView.Status = false;
authView.Message = message;
}
}
else
{
string message = string.Join(" | ", ModelState.Values.SelectMany(e => e.Errors).Select(v => v.ErrorMessage));
authView.Status = false;
authView.Message = message;
}
return View(authView);
}
This works fine. But when I keep the browser IDLE for like 30 minutes the "session_user" session variable get expired and the user still gets authenticated. How can I resolve this?
Also using cookie-based authentication can have a performance penalty?
Thanks