What you heard was correct. I’d even say that it’s fairly recently applicable too, considering that the most recent Gui engine optimisation I know of was made early last year.
I absolutely recommend that you separate your Guis. I once tried shoving my UI into only one ScreenGui but found that it’s simply not a good idea both for performance and organisation. Doesn’t flow or work as smoothly as I’d like it to. Being clear in what my Guis do has been working wonders (e.g. it seems silly to put loading screen elements in a menu’s Gui).
Ideally you will want to separate your ScreenGuis by the purpose of them. Static Guis (ones that aren’t updating) are significantly more performant. When a Gui element updates, this requires the Gui to need to be rendered. Using multiple ScreenGuis would isolate the performance cost into only rerendering certain changed Guis rather than all Guis.
Here’s a thread you can read up on:
I’d be willing to have as many ScreenGuis as actually necessary over using only a single. You’d miss out on the benefits that way.