More cores only benefits if more processes are running, or a single process is multi-threaded. Thus, when I say the 13" is faster for "general usage" I say so, because a lot of everyday tasks are single-threaded burst workloads.
Now you're right that if you do any work that requires processing to a larger extend than opening Safari or checking emails, the 15" is going to be faster though.
For anything that doesn't "care" about a beefy CPU, the 13" will be faster. For anything that does "care" (video, 3D, VFX, etc), the 15", no question. Which kind of begs the question: If your preferred apps don't care about needing a more powerful CPU, maybe the 13" MBP max config is overkill and you'd be better off with something lower-spec?