and younger Czechs have real problems understanding it.
Isn't this rather related to Slovak being a mess in terms of spoken dialects? I spoke to someone from Usti nad Labem a while ago about it, and she reckoned that while the Western Slovak dialect (or essentially TV language) was simple to understand because it was more or less similar to how they speak in Moravia, the messy Eastern dialects were much worse for her. I noticed it when I was there last year - I can get by in places like Bratislava and Zilina, but in Presov and up towards Svidnik/Bardejov, it just sounded like a funny Polish dialect and not like Slovak at all.