I'm also having WordPress slowdown issues. I signed up for Dreamhost this past Friday despite all the negative feedback surrounding their Data Center issues. Setup was fast and easy. I started working on the site via their dreamhosters.com mirroring service while the domain propagated. The Wordpress site was much faster than the host I was moving my client from so I was very happy and felt that I made the right choice. I duplicated all the content from the previous host, and since it was a new site for the client, there were only 5 or so pages and 3 blog posts. And I only had 3 plugins installed.
So Monday comes around and I started working on the site again via the actual domain name. Things seemed fine, but I started noticing a slowdown as the day went by. By the afternoon the Wordpress admin was very slow. The only reason the frontend seemed fast is because of the caching plugin that I had installed. But you could still notice the slowdown if you submitted a contact form. Any way, I submitted a ticket yesterday. Today the site was worse. I keep receiving 500 Internal server errors. Dreamhost replied to my ticket with the following:
"I've checked into why you've been seeing slowness/timeouts, and it seems your scripts have been getting automatically killed by our Process Watcher script due to your sites going over Memory limits on the shared server:"
I couldn't believe that. I really don't know what to say to that. Like I said above, this site contains minimal content and the blog is not even live yet. The site is loading a static homepage for now and there are no links to the blog.
Good luck with your blogs.