The app becomes unusable with large inboxes.

I am not sure if this is expected to work, but I expected it to work. I am not the only one: https://groups.google.com/forum/#!topic/k-9-mail/9F55x6i6Ssg
Mail in Opera 12 that is over five years old on a five year old laptop (top range, Thinkpad W530) works extremely fast on the same inbox.

Expected behavior

K-9 should cope with 10GB of data or 100 000 messages stored locally.

Actual behavior

When the Local folder size in Settings>Accounts>Fetching mail is set to unlimited with a large (around 70 000 mails) inbox, K9 downloads them all after crashing a few times but becomes extremely sluggish with Android frequently offering to kill it - and sometimes it crashes. It is slow to scroll messages and to display them (using split view, but I do not think that matters).

Steps to reproduce

1. Set the local folder size in Settings>Accounts>Fetching to unlimited with a large inbox (in my cse, 70 000 messages)
2. Wait a minute or two.

Environment

K-9 Mail version:
5.403
Android version:
7.1.1.
My phone is GeminiPDA - (this specs: https://www.planetcom.co.uk/?page_id=8 ) - it should not be too weak for this.
Account type (IMAP, POP3, WebDAV/Exchange):
Gmail over IMAP.

Start by reproducing the issue from Settings > Accounts > Fetching with an unlimited local folder and a large Gmail-over-IMAP inbox. Observe download, scrolling, display responsiveness, Android kill prompts, and crashes on the reported environment; done means the app remains usable with approximately 70,000 locally stored messages.