I think we put a lot less emphasis on vowels than most Americans do, particularly compared to those from the southern US (who are known for speaking slowly). I think that even people who speak standard American English sometimes stretch out their vowels somewhat, at least compared to us. Of course we speak slowly when we need to speak clearly, but in casual conversation almost every vowel can be shortened to the point where they almost sound like 'schwa' sounds. I think this can also be true for some American accents, such as from Boston or Brooklyn (that may be why some Americans think Boston accents sound similar to Australian accents).
Apparently people from Queensland speak slightly more slowly than other Australians; but as someone from southern Queensland living in Canberra, I often feel like everyone speaks more quickly there than in here.
People always say that the Australian accent started because settlers didn't want to open their mouth too far so flies wouldn't go in, but I'm not sure how true that is. I do know that the Australian accent was strongly influenced by both Cockney and Irish accents, but I'm not sure to what degree.
"I'm not sure about faster but I've heard that we Americans are considered fairly loud."
That's a common complaint about Americans in Australia; but then again, Kiwis say the same thing about us! :D
I'm not sure about faster but I've heard that we Americans are considered fairly loud.