SUMMARY
 If you highlight only a part (the most recent part) of the received mail and then, hit either “Reply” or “Reply all”, KMail behaves normally.
If, however, you Reply (or, Reply all) with the complete thread, KMail freezes.


STEPS TO REPRODUCE
1. Hit reply or reply all to an email at the end of a long thread that has been quoted.


OBSERVED RESULT

2. A compose window starts to open but freezes before the window is populated. Depending on the length of the thread the freeze may end after some time and allow you to edit the new email.
3.  If you edit in a previously frozen message, there will be lags, particularly on a new line or backspace.
4. If only the most recent part of the thread is highlighted before hitting reply, KMail performs normally.


EXPECTED RESULT
Normal new message window.


SOFTWARE/OS VERSIONS


Linux/KDE Plasma: OpenSuse 15.3 with kernel 5.3.18
(available in About System)
KDE Plasma Version: 5.23.2
KDE Frameworks Version: 5.87.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION