This may be a strange question. I'm looking to hire someone who can examine some load time issues I'm having on our VPS. It's not a server issue, but rather it seems to be a problem specific to some of our WP sites which use the same theme+customizations (they all tend to load in 10-15 seconds , but only on the first load after I haven't visited them for a while. Other sites on the same server always load lightning fast, and their total file size is larger)

I'm aware of tools such as wp cache + minify. I will apply these, but I think there is an underlying problem which needs to be solved first.

Now, my question is - what qualifications would someone who is also an expert in troubleshooting/possibly fixing load time issues have - who exactly am I looking for?