Quote: I would expect a closed-loop control to be more aggressive than that.

I agree. It may be a good time to remove and clean the IAC Valve, the tubing for it, and the throttle body. Also, seriously consider replacing your O2 sensor if it's older than 5 years.

how _quickly_ is the system supposed to react to changing loads?

Just about seamlessly. I do not notice the problems you mentioned in my vehicles. When shifting from "Park" to "Drive" and holding my foot on the brake, the engine idle drops maybe 100 or 150 RPM or so. I can't remember the actual numbers off hand, but I'm certain that it doesn't drop too low and then rise back to where it should be. Of note, my IAC Valve is cleaned once a year (and so is the throttle body, with it still on the vehicle), and the O2 sensor is only a year old. I also use full synthetic oil in my engine. This reduces (and almost eliminates) the amount of future deposits and varnish that would be formed from oil droplets blowing through the PCV system (since the synthetic oil stayes liquid and doesn't harden up like dino oil).

God bless,
Fitz Fitzgerald.
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'87 Blue 245, NA 236K