Thank you for posting this question.
This happens because the actions that you do are placed in a queue by ASP.NET. This means that when you execute certain task that takes time, the other operations are waiting for the ones before them (they become pending). The good news though is that every user's actions are in different threads, so your actions are independent to the other's users actions, and vice versa. This also means that other users will not have the slow down when you expand a page group in the admin. This is how Asp.net works.
On the other hand, it is not possible to separate the administration from the backend, for now.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items