I take contract work and I try hard to check in some speedups in every project I touch. It's not hard to find issues. Just profile, most people never do. The other trick of finding problem code is to talk to the sysadmins - they tend to know and often are not shy about criticising those awful lazy devs :)

I don't think you can offer it as a consultancy service because people tend not to care and are quite aggressive about that. They would much prefer to pay for more hardware and doing so is organisationally respectable: "the cloud will solve our problems".