Why is dreamweaver so slow?

Latency can occur when your local site folder contains an extrordinary number of files.  Try turning off Cache from your manage sites  panel.

The forum is pretty much instant for me. If you're experiencing lag there, it could be any number of connectivity issues with your internet, ISP, region, etc.

As for DW opening files slowly, I also don't have this issue in the latest version, though admittedly, I don't use it often or with all file types...

Does it happen with all files, every time, even new blank documents? If it doesn't happen with new documents, have you tried saving a new blank document to the same location as the rest of your pages and opening from there?

Where are your files being stored? Are they on the same drive as DW, network drive, external HD, etc?

Have you tried any of the standard troubleshooting steps like clearing the program cache, restoring preferences or reinstalling using the Cleaner Tool?