Case: LibreOffice 7 has a long (splash screen) loading time on a new Solus Linux installation. Installed via the Software Center.
Reinstall LibreOffice 7 using a different package. If the install is via the software center, install via Snap or Flatpak.
Another potential case is a faulty user profile. The issue is triggered by doing customizations to LO. This means that the app has loaded gracefully on first launch.
The provided solution to this didn’t fix the issue, but it might be useful for Windows users: Why does LibrOffice starts so slow? – Ask LibreOffice