Loading emails in the nextcloud mail app is very slow for me (10s+ on an average email), removing html and the encrypted elements from the vues makes it like 8 seconds faster but going with just the MessagePlainTextBody causes bad formatting.
Request Time (Never opened & Average length email):
thread- 94 ms
body- 2.69 s
html- 4.01 s
flags- 3.68 s
This is using googles SMTP and IMAP with their XOAuth as well, which should be a lot faster than this.