For the lag fix I would follow these instructions on how to install clockwork mod
Clockworkmod installation how to for windows / *nix - xda-developers, and download rom manager from the market place, then download the voodoo lag fix, and use clockwork mod to install it, it should restart your phone and run through the process for you. I strongly suggest doing a system backup through clockwork mod before doing anything to your phone.
For tethering, I use barnacle legacy. I tried the wifi tether app, and I couldn't get my laptop to recognize the phone at all, but it works perfectly fine with barnacle.