iCloud Mail Not Deleting - Very Slow - Have to Delete Multiple Times
This also occurs in Outlook for iOS.

It's an iOS threading (GCD) issue of one of two kinds, either:


GCD places low-priority on threads to which calls for "delete" are dispatched, and defers to any and all other operations on, like, any other thread; or,
GCD stops waiting for a condition to be met on the mail server side, suspends or kills the thread in favor of others, and then fails to resume or reinstate the thread at a later time.


It could also be improper implementation of GCD in a networking component of iOS, which happens easily enough for programmers with less than 20 years of experience with multi-threading behind them.