The only time ReSharper slows down your system is when you load your project for the first time. But I'm more than happy to wait for an second (we have ~100 projects in our solution) so I can harness all the greatness that comes with it. If you're not finding it useful, perhaps you're not taking advantages of the right features? Here's some of my favorite shortcuts, I'll let you try them on your own.
- ctrl+shift+l
- ctrl+\
- ctrl+r+r
- ctrl+shift+t
- alt+insert