I scanned some Kodachrome a few weeks ago, here are the results:
https://drive.google.com/open?id=1X3TQxCx1tps1nvkJdbuDsMR5yAaTcuLx
The photographer said he used high-speed Kodachrome.
They did indeed still look very good, but I have Ektachrome slides from that era with little or no fading too. Both developed by Kodak. I think it largely depends on storage conditions and quality of development.
How did Kodachrome 100 compare to Ektachrome 100 grain-wise? many people go on about how fine-grained Kodachrome is, but I think they forget this was at a much slower speed than the E6 they were comparing it with.
I don't remember Kodachrome with the speed you mentioned. I am not sure if there has been a Kodachrome with ISO 100 in the very old days - possible there was one but I doubt a bit. But the grain you asked is indeed an issue of interest. 3 speeds have been avaible in still film format.ISO 25/64/200. There have been ISO 40 Kodachromes in Super8 format. I remember the same film in 16mm. But it may be there was also one ISO 100 in 16mm???? I can't say
The grain was just excellent (with ISO 25) but there was one issue in addition : extraordinary sharpness.
Kodachromes with ISO 64 wasn't with such small grain character but it was ok in comparison. The ISO 200 Kodachromes were real grainy (just from my point) - no wonder if you notice the higher speed.
The ISO 40 motion films have been such perfect from smal grain and sharpness that most of super8 filming people were much much disapointed about the characteristics of Kodak Ektachrome100 motion film version.
Some companies offered Velvia50 in super8 format to that time - but Velvia50 can't reach the smaler grain size of Kodachrome40.
That's what many super8 filmers stated - and that's should also answer your question.
I personaly can't say in direction of Velvia50 super8 what about the grain comparison - because I never used it with super8.But I would believe that Kodachrome40 was with smaler sized grain.
A new E6 emulsion with ISO100 should have a very little bigger sized grain and little less sharpness in comparison to a Kodachrome100 film (just from my point) because I can't give you the evidence : I remember no test comparison in that case - but it seams to be logical.
Comming at last to 16mm motion picture version of Kodachrome.
There have been some alternates after discontinuation of Kodachrome : in kind of negativ films.
Regarding the excellence of ECN2 films - special in low speed version (ISO50) I would like to state : same smallest grain like Kodachrome 40 - perhaps a little smaller
so there have been no problem
from discontinuation of Kodachrome40 16mm films
.
But it was more expensive (not the Vision films itselfs) but you had to add the developement and the costs of a copy. (Kodachrome 16mm was inclusive developement and damned cheap in comparison)
with regards