Oh, so code that's entirely dominated by property loads will be much slower in Sparkplug than in TurboFan, by a factor of more than that 4×, perhaps closer to the 32× I was SWAGging for Ignition's overall relative slowdown? I guess at this point I ought to be running microbenchmarks instead of asking you to do it for me... ;)
Sorry, the 4x was my estimate of Ignition vs. Sparkplug (I guess it's closer to Iain's estimate of 2-4x). Sparkplug vs. TurboFan is crazy workload dependent. For microbenchmark-y tight loops, integer attorneys, and clean single-shape ("monomorphic") property loads, TurboFan is orders of magnitude faster. For small functions that just have non-inlineable function calls and heavily polymorphic property loads (with lots of shapes so you can't skip the property lookup), like you see in a lot of web code, they're close to on-par (because there's nothing TF can optimise away). If you put a gun to my head, I'd say it's on "average" maybe 10x? Very much a Fermi approximation though.
Oh, thanks! That's surprising! And precisely the kind of question I was interested in.