Solely from a performance analysis standpoint, you've just perfectly described something using 100% CPU across all cores. But you aren't reporting the phone be

Solely from a performance analysis standpoint, you've just perfectly described something using 100% CPU across all cores. But you aren't reporting the phone being warm to the touch, so... hm. Have you tried wiping and restoring it? It could be interesting to back it up, wipe it (and maybe remove the SIM card for completeness?), install the worst performing apps on the fresh install, and _then_ (maybe wipe again(?), then) restore it back to normal. If app performance is terrible in the fully-wiped state, I'd take the phone into an apple store (regardless of warranty status) just to see what they say. Hmm. Just thought of this: maybe your battery is like the OP's from reddit but in really bad condition, and your phone is downclocking itself to compensate?