What you are asking for is different things:
- Keep a large Website organized to make it easy to navigate.
This is ergonomics of webdesign, how do you structure the navigation, are you using a search form or tags?
This depends heavily on the type of website. Look at amazon.com. It is a huge website with millions of pages, but it mostly relies on searching. You can access categories and click through where you want to go, but if you know the name of what you are looking for, you will use the search and it will guide you.
If you run a website that is aimed to lead the user through steps, or provide information about a limited number of ressources - like selling a service that has three options - there is no need for a search at all.
Try to not have deeply nested navigation, separate only things that are really separate and group content in a meaningful way that is logical both for humans and search engines.
The goal is to allow a user to easily find their way in a few seconds.
- The other thing you are asking for is performance, which is a completely independent issue.
First of all, optimize all assets, all images, CSS, JS, concreteCMS will help you doing this, but much optimization is done on the level of the files (like images) direclty or via the webserver (caching time of static assets).
From the concreteCMS-side of things, try to use Full Page Caching wherever it is possible but mostly on pages that the user will step on first, like the Landingpages - they must load quickly. If the “Sign up Page” loads snappy is not too important. But looking at a blank screen when following a Google Suggesiton is a no-no.
One of the big slowdowns can be the Auto-Nav Block if you don’t use it carefully. It can index through all the pages within its scope which can punish the load time of a page - be careful to restrict its scope and it should be fine.
Using a CDN for assets will also help to speed up the site.
Considering those aspects, you will get quick loading times with a concreteCMS page of any size - because the complexity of the site will only be called when the pages are actually accessed.
If unsure, run your page through https://pagespeed.web.dev/ and you will see all the potential for optimization which is outside of the concreteCMS-scope like file optimazations and asset caching. Once you reach a satisfying score there and the site still feels sluggish, your leverage is on the concreteCMS-side of things like autonav.