Much of Dreamweaver's appeal centers around the Design View interface, which does a pretty good to very good job of rendering a page's CSS and HTML into a fair approximation of what the browser will display via the WebKit framework. Unlike a typical preview, web page authors can continue to edit the page content in this mode and, depending on settings, see placeholder icons for line breaks, positioned elements, code snippets, and much more.
Some users complain that Design View reacts so slowly as to become almost unusable and report having keystrokes take between two and three seconds to reflect on the screen. This can be due to one of several factors:
1) Due to the load of having to draw and redraw the layout and CSS styles as the author types content into Design View, Dreamweaver may become a little laggy. This information comes from David Powers (author of several books on Dreamweaver and one of Adobe's Community Professionals) in a slightly acrimonious Adobe forum thread. According to Mr. Powers:
Rules that apply to the <html> or <body> tags or that affect more than 10 elements force Dreamweaver to repaint the page completely.
His solution is to make sure your CSS declarations are as tightly targeted as possible and use descendent selectors to tie elements into their parent containers. This is certainly good practice but may come as a rude shock to users who upgrade to CS5 from CS4 and discover their pages are not as editable as they used to be.
2) Although the listed specs are fairly modest, in my experience Dreamweaver CS5 requires a fairly fast machine and graphics card as well as a decently fast hard drive and RAM. If you are running on a slightly older machine any or all of the preceding could slow the program down.
3) Using DW to edit pages on a shared network drive can really cause speed problems. We have three designers on staff and to make life a little easier, we pointed all of their local Site Definitions to a folder on a shared drive. The designers would complain intermittently that Design View would begin to lag throughout the work day. After a lot of hair-pulling, teeth-gnashing, and heavy drinking we finally traced the problem to network congestion on the Network Attached Storage (NAS) that they were using. If enough users hit the NAS, the constant redrawing caused by point #1 would eventually cause something of a bottleneck on that device and slow things down.
There are several possible solutions for #3. In our case, we moved the web design team to a different, less utilized, NAS and that has restored most of the functionality. A better solution would have been to enable Subversion integration (which is built into DW CS5) and have the designers use their local disks for editing. However, Subversion is not the easiest thing to set up nor use in Dreamweaver and if you are not familiar with it already, you may be a bit confused as to the changes it will bring to your workflow. For those of you in multi-developer environments who are willing to give it a go, please reference this DevNet article.
If you are having trouble with Dreamweaver CS5 and slow Design View response, I hope this article has helped you get things back on track. If not, feel free to post a comment here, or a new question in the Dreamweaver Zone and I'll be happy to try to help you further.