When I had ESET Smart Security installed, the Dopus window appeared immediately after double-clicking on the desktop. After uninstalling (end of license) ESET I started using Windows Antivirus. Now the window opens after about 30-40 seconds on the desktop.
If your Opus config loads toolbar/menu icons from lots of different exe/dll files, that might cause bad antivirus software to take a long time scanning all the files on each launch.
(Windows Defender is pretty bad these days, in terms of performance impact and causing more problems than it prevents.)
For a few months, I did not understand why the viewer took 5 seconds before being displayed.
I now understood by reading this post, I added 2 shortcuts to 2 software in the toolbar.
I extracted the icons from the applications exe file, just replaced the button icons and now the viewer opens instantly.
I learned that the hard way as well a few years ago after liberally using a 1 MB image as an icon that needed to be resized every time a toolbar was loaded 
Maybe that's something Opus could do automatically: extract the icons when the exe/dll is inserted and only update them on demand or when the host file changes.
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