Any film image is going to be scanned for print reproduction. Kodachrome can be more difficult to scan. If the slides are old and have dust on them then any attempt to use PP software to remove the dust will also lower sharpness. The qualities of Kodachrome are well known by now. The images made with it and printed in National Geographic did not look unsharp. What you are noticing about the images which were supposed to have been taken on Kodachrome tells you more about the way the magazine printed them than about Kodachrome itself.
The fastest Kodachrome sold had a speed of 200. I remember shooting Kodachrome II in the early 1970s. In some cases a tripod was needed. More hand held shots with slow film are taken at or near full aperture. This reduces depth of field and in most cases absolute sharpness. Today we have very high quality fast color films like Portra 400 and Portra 800. This wasn't always the case. The recently discontinued 100 speed Kodak Ektachrome slide films have very high image quality and are two stops faster than the Kodachrome 25 which was discontinued not so long ago. Apart from selective focus applications, very fast lenses like the Canon 85/1.2 and the 35/1.4 Nikkor were made to allow more use of slow and fine grained films. The very high ISO performance of the current crop of top DSLRs was not yet available.