> It's mostly the dialect (pronunciation), fast speaking and unclearly articulated or swallowed sounds.
I agree. I was once in a car with a Brit and Canadian and had a lot of trouble to understand them as a non-native English speaker. They could understand each other perfectly well. So, it must be not the specifics of the British or Canadian accent, but more about the fast speaking like you say.
True, dialects differ. My father was a cockney in London and my mother was Irish, we lived in Wales during WW2 and my father was a Major on the Burma Road - came back with a love for curry that the cook soon adopted. We often had Canadian and Australian officers stay with us.
The net result is I am fluent in all British dialects, and adapted to the Canadian dialect in school - kids who spoke funny soon learned Canadian-speak.
Once, on the phone, my mother asked me a question, and back I went to Oxford English to answer that question - when I returned to my call, the called asked me who was that? I had switched back to UK-speak and back to Canadian-speak instantly. I also learned Yiddish - my father was a stained glass artist, and while he was in Burma the home office asked him if we could accommodate a family from Poland who had given my father as a reference. It was Mr Berger, my father's art glass supplier in Poland who had escaped via Spain as a DP family. Father and mother, 2 children and their grandmother - we had the space and my father agreed by telegram. My mother and me at 4 were immersed in a Yiddish family. It was great, I soaked up the language like a sponge. In 1946 my father returned and the Bergers went to live in a house they rented.
In 1948, we came to Canada. Many of the storekeepers in Toronto were amazed at my Yiddish fluency, as we often shopped in Yiddish store - no supermarkets then, we shopped 2-3 times/week. Butchers, fishmongers, bakers, grocers, greengrocers all a melange of differences - I still long for these aspects as I wander Costco these days.
Try Glaswegian. I'm from the south of England and on occasion hitch-hiked in Scotland. Got a lift from Aberdeen where they speak perfectly understandable English to Glasgow with a Glaswegian building contractor. I don't think I understood more than half of what he said along the way.
We once had an employee from Westinghouse from the Netherlands over at our house for Thanksgiving. The poor man could understand everybody in the family except for my grandfather. He finally asked us what he was doing wrong. We all laughed and simply pointed out that the problem was my grandfather's accent, not his English and that a significant portion of America wouldn't be able to understand my grandfather, either.
I agree. I was once in a car with a Brit and Canadian and had a lot of trouble to understand them as a non-native English speaker. They could understand each other perfectly well. So, it must be not the specifics of the British or Canadian accent, but more about the fast speaking like you say.