True, having accurate indexing motion is a different thing than having responsive shifting.
The difference seems to come down to how well that the vintage derailer keeps the chain gap between pulley and cog small enough so as to effect shifting response using typical modern (i.e. flexible) chain.
The vintage friction derrailers were designed before truly flexible modern chains were created, as even the Sedisport bushingless chain was/is far stiffer laterally than any modern chains available today.
Vintage derailers may lack any b-tension screw needed for adjusting the chain gap, and vintage Suntour derailers having a B-screw still may have excessive chain gap with the screw removed entirely!
Vintage Simplex derailers usually feature a cage-spring tension adjuster nut that can be loosened to change the spring tension then re-tightened, this alters the spring tension balance between the mounting pivot spring and the cage pivot spring so as to adjust the chain gap.
Suntour friction derailers can have the chain gap further reduced (after removing the b-screw) by carving a bit of metal from the screw boss, for more-responsive shifting in both directions.
Dual-sprung-pivot friction derailers like Shimano, Simplex and the Gran Turismo(!) can have their spring tension balance altered by means of drilling new hole locations for the end of the cage pivot spring, as shown below. Changing one spring's tension alters the chain gap just as if the derailer had a b-tension screw!
Lastly, good shifting performance, whether indexed or friction, also depends heavily on best cabling performance, with all cable paths lined with plastic noodle (even at the bb cable guide), and with all flexible housing junctions made not to move around in response to shift lever movement.
One more and final thing that helps the shifting response of typically lazy-shifting friction derailers would be using only a
non-floating top pulley (and making sure that the pulley bushing and teeth are not heavily worn). Pulley float is NOT needed or desired when a typical old friction derailer already suffers from an excess of chain gap!
