To speed up email address suggestions when composing an email, go to menu > Settings > Mail > Compose and disable “Use all recipients from Sent folders for suggestions” under Email Address Suggestion.

Depending on your email history, this option can cause a significant delay since the app needs to load recipients from Sent folders across all your added accounts each time you're composing an email.

Disabling it should resolve the issue, but if you still experience slow address suggestions, you can also try using the Clear recipient history button or deleting specific irrelevant addresses via the Show recipients button. If you only want to use your Contacts for the autocomplete, disable “Store composed mail recipients for suggestion”.
Read more on this feature on our blog.