.NET Core alternative to ThreadPool.QueueUserWorkItem

I'm working on implementing the ForgotPassword functionality ie in the AccountController using ASP.NET Identity as in the standard VS 2015 project template.

The problem I'm trying to solve is that when the password reset email is sent, there is a noticeable delay in the page response. If the password recovery attempt does not find an existing account then no email is sent so there is a faster response. So I think this noticeable delay can be used for account enumeration, that is, a hacker could determine that an account exists based on the response time of the forgot password page.

So I want to eliminate this difference in page response time so that there is no way to detect if an account was found.

In the past I've queued potentially slow tasks like sending an email onto a background thread using code like this:

ThreadPool.QueueUserWorkItem(new WaitCallback(AccountNotification.SendPasswordResetLink), 
notificationInfo);

But ThreadPool.QueueUserWorkItem does not exist in .NET Core, so I'm in need of some alternative.

I suppose one idea is to introduce an artificial delay in the case where no account is found with Thread.Sleep, but I'd rather find a way to send the email without blocking the UI.

UPDATE: To clarify the problem I'm posting the actual code:

[HttpPost]
[AllowAnonymous]
[ValidateAntiForgeryToken]
public async Task<IActionResult> ForgotPassword(ForgotPasswordViewModel model)
{   
    if (ModelState.IsValid)
    {
    var user = await userManager.FindByNameAsync(model.Email);
    if (user == null || !(await userManager.IsEmailConfirmedAsync(user)))
    {
        // Don't reveal that the user does not exist or is not confirmed
        return View("ForgotPasswordConfirmation");
    }

    var code = await userManager.GeneratePasswordResetTokenAsync(user);
    var resetUrl = Url.Action("ResetPassword", "Account", 
        new { userId = user.Id, code = code }, 
        protocol: HttpContext.Request.Scheme);

    //there is a noticeable delay in the UI here because we are awaiting
    await emailSender.SendPasswordResetEmailAsync(
    userManager.Site,
    model.Email,
    "Reset Password",
    resetUrl);


        return View("ForgotPasswordConfirmation");
    }

    // If we got this far, something failed, redisplay form
    return View(model);
}

Is there a good way to handle this using other built in framework functionality?