I'm an expert in a particular piece of software, namely Dynatrace, which is an Application Performance Management platform. I can see that "performance management" is a skill I can specify in my profile, but I can't see a top-level category that I can use in my profile -- in the pull-down that says "what are the main services you offer clients". The real answer is, "help with configuring and using Dynatrace", but that's not one of the options :-).
To avoid getting into the specifics of performance management, load testing, etc., imagine I was an expert in Excel. What top-level category would I use in that case?
Dynatrace! Used it at Citrix. Great program. They use it at Bank of America too. Huge learning curve though.
I would put yourself under programming/IT, personally. For something like that, it's gonna be an enterprise level IT operations and development team looking for someone to set it up so they can monitor things. You're random schmoe working in his mom's basement won't have something like that.