That suggests something outside of Opus, since it's slow once and then fast the next time, when Opus would be doing the same things at startup in both cases.
Might be some part of Windows that the first run causes to initialise and is then faster the second time it's used.
Could also be something like virus scans of something Opus accesses (wouldn't be Opus itself, as that would happen before it opens the window), or spun-down HDDs spinning up, or similar. Along the same lines, if something in the toolbars/config or initial folder points to an inaccessible network drive, that can be slow to timeout the first time and then may be cached by Windows as unavailable (at least for a while; eventually it'll be slow again).
Process Monitor logs of the slow startup may point to a cause, if you want to try those.