Hm... "most cases". What kind of "most cases" are you thinking of? Have you used Class::DBI on a heavily loaded web site, for example?

"most cases" == most websites. Most websites are not heavily loaded. But even on heavily loaded websites, it's still usually a minority of the code that gets hit the most.

Do you have any suggestions on how you would find out when it matters? Any hints on profiling a Class::DBI app?

Find the most visited portions of the application (that's what server logs are for) and then profile it just like you would any normal web application. Like this for instance.