There is a number of fixes and optimisations that we have found and applied for v10. A few SQL queries have been rewritten, especially in Timeline and Notifications. Those are hard to find with simulated load tests because they need actual push-notifications subscribers and in some cases interactions we couldn't predict. 

So, we have worked with one site, for example, which is getting up to 700 online users at peak times, and growing. So, to answer your question, yes, UNA can handle a large load, but it is important to set up caching properly, apply latest version updates and continue monitoring page loads - some issues may be specific to the site and we need feedback on what exactly isn't performing well.

Another important consideration is to not overload your site with JS libraries (like analytics, chats, marketing popups, ads, etc) or at least ensure that they are loaded asynchronously. 

We will be sharing more speed optimisation tips along with the v10 release. Performance is the top priority for us now and the next update will bring significant improvements.