iMac 2017 i7 performance after upgrading memory

about a software then can really test memory performance

There is one that comes with every Mac. It's called Activity Monitor. As long as the memory graph is in the green there's no memory issues to worry about:

Never install any app that has "Dr." in it's name. lol

If you have real time scanning running in Malwarebytes that could slow things down some.

But what makes you say it's "underperforming"?

IMHO, people waste more time measuring their computers performance than actually doing something with it.

There's nothing wrong with Malwarebytes per se. You can use it when and if ever necessary manually. It doesn't have to run in the background.

You can post a EtreCheck report so anything obvious will possibly be seen.

I don't know what "become heavier" means. I've owned computers for more decades than I care to say and never once have I been tempted to "check the limits".

Adding more RAM won't make the machine any faster. It just makes it possible to run more concurrent applications without that slowing down performance.

Not likely Unless there are other issues with the computer. Any anti- virus, cleaning or other “maintenance“ apps installed?

Amen!! 😎

That is the Unix kernel process that provides all the low-level routines needed by running programs. The more kernel routines being used the larger that task will become.