What is the speed of your machine like? I have been working with an application with many hundreds of views without too many problems. I've experienced the designer client running significantly slower on some older machines though...
Alternatively, could you try working on a template of the design locally to save on trips to and from the server?
I see you are running 8.5.2, if you are on less than 8.5.1 then the designer client is a lot less responsive.