Not sure why this is so in your case but in general Dreamweaver needs to check the validity of your software by looking at the serial number and whether it is activated or not. If you look in the Process Inspector of your system, you will find that there are so many processes running in the background and these all have to load before loading DW.
15 seconds may be a lot to wait but on my old Windows XP it takes about a minute to launch it (this is whole 60 seconds or 4 times 15 seconds). That is why I prefer to use CS4 on XP machines.
PS: this message took 7 hours to post after your posting!!