There's nothing intrinsically rude in asking someone to slow down, because of an unfamiliar accent, hearing problems, or whatever else.
I live in an English-speaking country which has a very different accent to any of the ones I grew up hearing. Worse, the majority of people speak pidgin (this is how it's described officially). They generally don't pronounce the last consonant of words that end in a consonant and the word order is far from standard. Articles and many prepositions are omitted entirely. And a number of common words are replaced with the equivalent words from one of the many neighbouring languages. "Can I eat now?" would be "makan can?", "are you sure?" is "izit?" (originally coming from "is it"), and "what do you want?" is "you wan wha?"
This alone would be difficult enough, but the extremely rapid rate of speaking means that I struggle to understand a lot of people. If I need to understand what a salesperson is saying in order to purchase something, I'll say: "I'm terribly sorry, I'm somewhat new to this country and can't quite understand the local accent yet. Could you please slow down?" So they'll repeat it again at the same rapid pace. Speaking fast is very firmly rooted in the culture
I once asked a lady eight times
to please slow down. Each time she looked at me sympathetically but repeated what she'd said just as quickly as before. In the end I just said, "sorry, you're not speaking slowly enough for me to understand. I'm sorry, I'll have to find someone else who can help me understand them." She had no problem understanding me, but for all I know she was speaking gobbledygook.
For 70-80% of people here I'm fine. It's just those with the stronger accents that I struggle with. I'm getting better, though. But the fact that people seem to actually be incapable of speaking slowly is frustrating. English is their first language, so it's not an attempt to overcompensate by hiding errors. The lady above lost a very big sale (over $1000) because I couldn't make out a word she said, and she couldn't slow down or give yes/no answers. This isn't unusual, unfortunately. But fortunately, all the news and 'standard' television is in British English, so they can understand me just fine. At least it doesn't go both ways.