This occurs in org-compat.el:
(eval-after-load "flyspell"
'(add-to-list 'flyspell-delayed-commands 'org-self-insert-command))
By not using a package symbol, this code runs whenever any file named
flyspell.el is loaded. Guess what one of the :checkers spell module's
autoloads files is named? flyspell.el
spell-fu lacks support for multiple dictionaries, affix expansion, and
many non-English dictionaries, so I've added back flyspell support to
the spell module, but opt-in, because it is still the significantly
slower option and spell-fu may support them one day.
If not, tlikonen/wcheck-mode is another alternative to consider.
Fixes#3813