Running processing functions in background

[Jean Louis]

I have some functions that are blocking Emacs interface
while they are processing database in background. Functions
are not interactive. Output goes to dedicated buffer for
later human review. And I prefer not to use external async
package.

I would like to have responsive Emacs while running and the
background function I think to intertwine with
`run-with-idle-timer' as function is sending emails to the
mailing list and there is no rush in doing so.

The `make-thread' did work well but interface remain not so
responsive, it slows down.

Is there some other approach when using only Emacs built-in
functions?