Yes, sadly Factorissimo does take up a lot of processing power. If you were playing singleplayer on a machine with those specs, you shouldn't actually have experienced any kind of slowdown yet with only 40 factories. However, if you are playing multiplayer, remember that each player's PC needs to simulate the entire world by itself (since that is how Factorio multiplayer works inherently), and so your world will run only as fast as the slowest computer involved can simulate it, because the other computers will need to wait.
If you need to loosen up your framerate, I recommend taking your mass production out of factories, i.e. things that you need more than one or two factories' worth of space for. A few factories should still be fine though.
I would appreciate if you give me a screenshot of your world with show_time_used_percent enabled in the debug menu, preferably on the slowest computer.
