Even 15 seconds is very a long startup time.
What have you tried so far?
Have you tried using Process Monitor to see which files dopus.exe is accessing during that time?
Tried making Opus open a different (and empty) folder to usual, in case the problem is triggered by something in the folder(s) it normally opens?
Tried disabling viewer plugins and shell extensions? (See this guide for info.)
Checked for disconnected/unavailable network drives?