By default, WordPress loads login page with the “Registration Complete. Please check your e-mail.” message, but sometimes it takes way too much time for the user to receive activation email or it is detected as a spam mail.
It’s a better idea to redirect users to custom page after registration. That page could be a Thank You page or a page to notify about the confirmation email issues and stuff.
Just add this following snippet to your current theme’s functions.php file:
return home_url( '/custom-page' );
add_filter( 'registration_redirect', '__my_registration_redirect' );