Delay before user object defined

Hi
I recently moved my site to a subdomain and am referencing it from another website. I have a link from the main website to the login form on the subdomain. Upon logging in, the user is taken to the WordPress toolbar screen so I know the login was successful. I have a page of custom code (that has worked for 2 years) and when I go to it, the user object is not defined — the function ‘is_user_logged_in()’ returns false. If I wait 45 seconds (and do nothing else) and refresh the page, the user is logged in.

Interestingly if I go straight to the subdomain and login there is no lag before the ‘is_user_logged_in()’ function returns true.

Any ideas how I can speed up the 45 seconds? Or what might be causing the delay?
Thank you!!