This is good advise, listen to dan. WP Super Cache is amazing although getting it working just right can take some tweaking.
The Redis Object Cache plugin is worth a try. It'll only take a minute to set up.
Is it 200k users or 200k page loads? Those are really different as each user will load multiple pages in a month. If it's 200k page loads then that server is way way too powerful (and expensive). Don't let a crappy developer hide their lack of optimization skills by throwing your money at the problem.