Resharper Ultimate huge input/ui lag when editing large code files (vs2012, C#)

One project I am working on has some pretty huge spaghetti-code files with like 20k lines of code. The fact that it is this way is beyond my control to change. Turns out that if I suspend Resharper editing things is actually really fast. I can't tell the different between editing a document that is 500 lines or 5000 lines. But with Resharper ON it basically becomes unusable. Eg, copy something, paste it 5 times, each paste takes 5 seconds to actually complete.