iOS iPhone/iPad Mail - rapid mass delete/delete all
Late edit
Not sure when this appeared but there is now [in 13.7] a simple 'Select All' button, like we always wanted. Follow that with Delete, accept the 'are you sure?' message &amp; at last we have a simple solution. ;)

Scroll to the top or bottom of your list;
Tap edit, then swipe up or down the circular icons [the checkboxes] until you reach the other end.
It will quite rapidly auto-scroll once you reach the limit of the current view.
Tap Delete.
Gone.
100 unwanted emails, 3 seconds. Gone.
Discovered [by accident] on iOS 13.4.1