In order to get a real impression of what Mendix is capable of i would suggest contacting Mendix and arranging a demo session. But in the meantime there are several public applications that you can peruse to get an idea of the type of applications we can build and judge their performance. Here are a few of my favourites:
All these sites are fully built in the Mendix technology
If anyone else is looking for great examples of UX/UI in Mendix then please check out our new App Gallery:
It contains several example applications to help inspire developers as to the art of the possible with Mendix.
One place to start might be to look in the App store (https://appstore.home.mendix.com/index3.html). Follow the menu link at the top called Apps which will show you a list of a few Mendix-based applications that some partners have developed as COTS. There are clearly several thousand other applications that organizations have developed for their own use or for their clients, but this list will give you some with a description of their uses.
The issue with this is that most applications require a login to actually view. I am currently working on the eXp realty enterprise project which does have a limited amount of pages available for visitor users.
http://www.expenterprise.com (runs in the public cloud in the US)
However, the majority is behind the login that I can't provide you with for obvious reasons :) I suggest you contact MX account management and ask if they can arrange some demos.
There is a quick video here of the GDPR app I am about to release, shows a small fraction of the capability http://gdprmentor.com/howto