So I'll preface that I'm by no means an expert. The issue can occasionally be the user's email provider but for the most part issues with implementing one-time

So I'll preface that I'm by no means an expert. The issue can occasionally be the user's email provider but for the most part issues with implementing one-time links will be the site's responsibility/a problem on their end. As for email occasionally being super slow, Google and Microsoft (to a lesser extent) will tarpit(repeatedly delay the acceptance of emails from a specific address or entire domain) emails arbitrarily. This normally isn't anything meaningfully repeatable and is essentially ~~the algorithm~~ arbitrarily deciding it dislikes some certain email, email address, or server. Those emails will eventually get to their recipient but effectively get frozen in time for a bit before getting delivered. For normal person to person emails, this boils down to bad luck and occasional inconvenience however for any heavily templated email (like say an email containing one-time links), this means something on the site's side is causing the emails to appear as spam or spam-like to the recipient server. So technically it is the user's email server causing the delay but it's usually due to something in particular on the site's side be it a domain or server configuration or something about the email contents.