One thing I hate about IAC is how slow it is to return to low idle. (On all cars with EFI.) I know it's to resist stalling, but I like an engine to crisply rev, then instantly return to idle.
So this is a part of tuning. Three factors affect idle speed. Spark advance, fueling, and air intake. For older ECM's the timing affected idle fastest, fuel next, and IAC last. Many of the OE vehicles are programmed to decelerate more slowly. Both emissions and OE needing "vanilla" tuning play a part in that. Newer ecm systems, and systems with electronic throttle control, can drive IAC and idle speed changes very quickly.

with the old slow ECMS i had usually run open loop, disable AE, lock down timing outside of operating range, and just do my best.
When I built the 302 in my '57 in 93, I was told the 350 hp 327 copy cam I was running was too big. I was using an 83 Camaro ECM back then and my only paid for tune from Turbo City ran like poop in OL. But CL wasn't bad, all things considering. Very wide INT and BL limits combined with mild VE changes were all that was done. I was very disappointed with that tune considering I worked for GM dealer and felt the vendor had promised it would run well. After learning to tune I had to give the tuner props as he worked entirely from paper specs and only got one shot to get it right.