That's because it's a powerful program. The more functionality you get, the more load it's going to put on your system resources. For instance, one design program I use often is lightening fast on just about any computer, but that's because the program is Notepad. NO functionality equals a very fast program. If you want something that will actually ease your coding burden, it comes at a cost.
IMO, amidst all the WYSIWYG editors out there, DW is the most customizable and, therefor, the most useful.