It might be better off in 4.0 than 3.x. Honestly the only difference it makes as it relates to code is it adds each class individually to the class maps in Composer's autoload_classmap.php and autoload_static.php files, so when PHP has to get into the autoloader Composer will check that map first then fall back into scanning the filesystem based on the namespace. So for our uses right now, the performance gain is probably negligible, but in a proper Symfony or Laravel based project which is entirely Composer driven it can make a world of difference.

As we have only external packages loaded trough composer, guess it will be worth a try to run it in optimized mode.

From https://getcomposer.org/doc/articles/autoloader-optimization.md#trade-offs:

There are no real trade-offs with this method. It should always be enabled in production.

So from what I understand, Level 1 optimisation should be done as there is no reason not to do it. Level 2 is another topic and doesn't look like suitable for us.

I was playing around with it and it creates bigger classmap files. Created a pr for it #16896. I think it is worth to enable it, even tough in staging the impact will be little, but in J4, where we have more libs installed, we will get some improvements.

Closing as we have a PR :)