This was my experience as well updating computers at a company I recently joined. Changing to an SSD had the most impact, although a lot of these machines already had 8GB of RAM. There are still a handful around that now have an SSD but still 4GB of RAM they came with; these are also better now, usable anyway.

I did this to all my family members macbook pros a few years ago, back when apple let you do that sort of thing to your hardware. These machines are 10 years old and still running modern stuff just fine.