I have 4 themes and 350 storys (I´m filtering out completed ones, about 50)
I use Chrome, latest version on  a Windows 8.1 OS with ok hardware... 

I still think that the site is pretty slow when loading my backlog. I have only used this tool for a couple of months and I dont think that the number of storys in my backlog is that high. 
Are you working on perfomance? 

As it is now I thinking of finding another tool to use...