A modified version of the default maximize kwin effect that:
Wait some milliseconds for the soon-to-be resized window to arrange their new content.
If the destination size is much greater than the source size, then smoothly fade-in and resize to the new window size.
If the destination size is close to the source, then just do a smooth resize.

You can install via Systemsettings (effects window).
...or manually:
* Download the archive
* extract the folder present in the tar.gz into your ~/.local/share/kwin/effects/
* from a terminal, start kbuildsycoca5 (or restart kwin, or relogin, or reboot)
* open the effect panel and disable the original maximize effect, enable maximize for xrender

*** BIG FAT WARNING ***
Disable the default kwin maximize effect, don't run them togheter.