When you change the setting in the web panel to add or remove 'www', it changes that setting in the apache configuration on our web server. Ideally, your setting should match what you have in your WordPress settings for the URL, and that can help overall performance. Setting them opposite each other on the other hand, creates an infinite redirect loop.
The Page Speed Optimization option should be used with caution— while it does help speed up some sites, it can do the opposite for others, depending on any conflicting processes that might be in place. Also be wary of custom theme processes and outside elements that need to load with the site. Of course, the WordPress install and all themes and plugins should be kept up to date, and you might also try using PHP 5.5 or 5.6 to see if it improves things.
You can also make subtle improvements to your database tables in case any mysql queries are moving too slowly:
We have a few tips here for optimizing WP in general:
http://wiki.dreamhost.com/Fine_Tuning_Your_WordPress_Install (also includes a plug for our DreamPress service)
I also noticed the servers you're currently using are in one of our west coast data centers; we'd be glad to offer you a move to one of our newer SSD servers in our east coast data center; both the upgraded drives and the shortened distance to you and your visitors should make a noticeable improvement. If you're interested in that go ahead and submit a support ticket and we'll get that started asap.
Hope this helps!