Even then they were pretty slow. Slo-mo zombies. Except when they had flares or torches in their faces, then they threw up their hands pretty damn fast. As an older, more experienced movie viewer, I notice all sorts of things I didn't when I was younger. It makes me a better filmmaker or actor in theory but it makes me a miserable movie viewer. The only reason I haven't given up on movies altogether and just read books is those surprising rare gems that come along when you'd least expect them to.
"I like simple pleasures like butter in my ass and lollipops in my mouth." - Floyd Gondolli
It's not that they're slo-mo zombies, it's that others are fast-mo zombies. As an older, more experienced movie viewer, surely you've noticed the decreasing patience and attention span of movies in general over the years.
Points to the slow yet certain grip that death has over all of us. Romero zombies will get you eventually. Running zombies will snap tendons and break bones as they push through any living pain barrier. They don't have the same longevity. Modern zombies are short term terrifying, but can be outrun in the long term.