SSME vacuum ISP 452 seconds. Beyond the ability of humans to manufacture at design time. This had a predictable impact on construction cost and delivery date. Beyond the ability of humans to make reusable for a decade after deployment. Eventually turned into one hell of a monster legendary engine, a few decades later. Nothing wrong with an engine development program, but the SSME was ridiculous.

Merlin family vacuum ISP a sedate 311 seconds. COTS and evolutionary based on existing engineering practice. Designed for low cost and reliability, not utter extremes of high performance.

(edited to correct, not COTS. Kept thinking they adapted a RD series design at the start, but I checked and they didn't. Same general story though, lower Isp and lower chamber pressure and very conventional pintle injector design etc its good engineering aka very boring, all their engine design competitors are far more competitive, which is dumb because fuel and tanks are cheap, engine development isn't)

Its very much like comparing the long term reliability of a 2024 specs F1 series racing engine none the less manufactured today in 2014 (or at least they did their best), against the engine in my new Toyota that's in it's 10th mfgr year and is pretty much bulletproof.