You can change settings to make email processing and viewing faster. Speed can be impacted by the available device RAM when you have a large number of emails. (20,000+)
Normally AltaMail will only display the first group of approximately 1000 emails. You can tap on the "More..." or "All" buttons at the bottom of the list to load more emails. There may be an initial loading delay for 10,000+ emails.
You can reduce the number of emails being downloaded from your email server in the edit account settings (View). Tap on the Settings section. Then turn on "Max age (days)" (IMAP) or "Max emails" (POP). eg. set maximum age to 60 days.
Other apps restrict the email count automatically. AltaMail gives you control of these settings.

You can also change the "Max age (days)" or "Max emails" in the email folder settings when you view a single folder. This is in the action menu. (View)

You could turn off the email history to speed up email processing. More

There are also settings to preload email message content which will make viewing emails quicker but will take a little more time to download each email when it first appears. (View)