My visual studio 2012 became really really slow when editing ascx file. In entire of my solution there was only 1 web part and bunch of other cs and other important files. I noticed when I am editing ascx file it was really really slow and unresponsive after every couple of seconds.
I found that when you have some urls that uses //somescript.path.js instead of http:// or https:// to allow browser to auto resolve protocol, that is happening.
After reading this somewhere on web, I changed my code to use Request.Url.Scheme in order to dynamically choose protocol instead of using // in the starting and problem was resolved.