Not sure what heavy to load means. An assembly is loaded once and stays in memory while the app is running and serves every request from that point, provided the server is not starving for memory. I do know Telerik.web.ui.dll is about 19M on disk now and growing and the computer will load it, hopefully, in less than a second.
I was reading a discussion on why they decided to put everything in a single dll. There are pros and cons to this design. The question then becomes do the pros outweigh the cons.

Anyway when it comes to performance tuning there are many areas in DNN to investigate before I blame Telerik's controls. I am not sure how much markup their controls create. That's would be my concern. It's the amount of html, css and javascript they create to render one UI control.

odd, the articles there but can't be loaded, it's either some exception or an over enthusiastic url rewriting rule (most likely explanation). In short, the article outlined that telerik and DNN Corp's agreement came to an end and would not be extended (for a number of reasons, not least Telerik have competing products). As such the telerik.web.ui.dll we ship will not be upgraded for latest releases - however we have an arrangement where by DNN Corp can make minor changes to fix user-agent related bugs (e.g. bugs introduced with IE11 etc.) and telerik will rebuild us the dll. This means you should not expect future feature upgrades, and would be better to minimize or stop your usage of the telerik wrappers - but that you are protected against future user-agent related bugs to allow plenty of time for people to gradually move off it.

---

Buy the new Professional DNN7: Open Source .NET CMS Platform book Amazon US