iOS 18.2 Mail app not updating in real time

How often your mail app updates depends on the type of account that you have and the settings you have for your mail account. Only iCloud and Exchange accounts will do Push to get mail, which is real time. Other accounts are usually IMAP, although there are some that are still POP. Those accounts will follow the Fetch schedule that you will set in the Mail settings. The most often you can check for mail on those accounts is every 15 minutes, no more often than that.