Slow mail compose with encryption, signature, and large attachment

Expected behavior

Since modern smartphone's processor is fast, few seconds should be enough to actually encrypt/sign attachment and message itself.

Actual behavior

After writing message and attaching attachment, K-9 takes about a minute to finish composing message and go back to message list with 'sending' notification on actionbar. It finishes sending in acceptable time considering my network condition.

Steps to reproduce

1. Attach large attachment (> 5MiB) to mail.
2. Select encryption + sign.
3. Press send button.

Other informations

OpenKeychain file encryption output (identical 5.4MiB JPEG file) with ASCII armor, compression, signature takes 2 seconds.

Environment

K-9 Mail version: 5.403 (downloaded from Google Play)

Android version: 7.1.2 - LineageOS 14.1 (Oneplus 3T), 8.1 Stock (Nexus 5X)

Account type (IMAP, POP3, WebDAV/Exchange): IMAP/SMTP, but I don't think this is related to this issue.

The issue names no source files or tests. Start by reproducing composition with an attachment over 5 MiB while encryption and signing are enabled, then trace the compose/send path and compare it with the reported OpenKeychain timing. Done means composition no longer takes about a minute while the encrypted, signed message still sends correctly.