That sounds excessive. I have noticed that with two identical MacBook Pros, one used for work that has mandatory anti-virus, and the other used for home personal without anti-virus, Apple Mail loads much faster on my home device. But even the "slow" work device opens Apple Mail within 10-20 seconds or so and once open, individual messages open quickly.
Do you have any anti-virus or third party security or optimizer ("cleaning" or other) software tools installed?
I suggest you download Etrecheck and run it allowing for full disk access, post the output here with the Additional text button below. Check the output, it has no personal information, but it does have configuration information that could aid readers here in identifying the culprit.