I have been hosting on DreamHost for a while now, and the fact that the actual servers are located in the States has never been an issue. Most of my site visitors are not from Australia, in fact the majority originate from the USA, so having my sites hosted there is actually an advantage.
Speeds have been mostly fine since I've been here, with only a month or so when things slowed down due to network problems at DreamHost. These issues were rectified and speeds have returned to normal. Of-course, some individual servers still suffer from speed problems here and there, but, in my experience this occurs with all shared hosting. Overall I would rate the speed of DreamHost hosting as very good to excellent.
The web admin panel can be slow(ish) at times, but never to the point of not being usable. I don't think this is related to any extra latency caused by me being in Australia. I think it has more to do with the large numbers of customers DreamHost now has. I believe there is a team dedicated to improving the panel response times, so I expect this to improve as they implement changes over time.
If you have any other questions, ask away
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