Some have noticed that Dreamweaver can perform sluggishly.
One reason is that, by default, it's checking all files in a site (and all subdirectories) to determine if they have changed--and it does this every time you leave and return to DWMX, such as to go read email, edit a word processing document, etc. The thinking is that you may have created or edited a file while outside DWMX. Unfortunately, that "lookup" can cause a painful delay if you leave and return to DWMX often, especially if the site has many files (and subdirectories).
The option is controlled by the feature "refresh local files list automatically", a checkbox in Edit Site>Advanced>Local Info. Turn it off and see if that helps.
You can use the "refresh" button in the site tab to refresh the list. Also, if you expand a directory for he first time since opening the site, that will also obtain a refreshed list of what's in the directory. Once it's opened, though, the file list is cached and only using the "refresh" or restarting DWMX will obtain an updated list (with the "refresh automatically" option disabled).
It's worth clarifying that this is a site-specific feature. You can leave it enabled on one site and disable it on another.
One other comment about poor performance in DWMX in general: I mentioned in my entry of 9/27 ("Getting into Dreamweaver for Studio users") that your machine's performance may have an impact, and in an entry on 8/10 ("Time for developers to consider a hardware upgrade?") I suggested other benefits of considering a machine upgrade.