I haven't noticed any slow down. I was demoing the capabilities of the Surface 2 today for a co-worker and was putting it through its paces... playing fullscreen video on an HDTV while navigating and working on the Surface 2 screen... very smooth, no lag or hiccup. Loaded up apps like News and Weather while all of that was going on. Still very quick.
Could it be network-related? If possible, you might want to try accessing those apps when connected to a different network.