Sib 7 introduced a very large music library. Terrific idea--poor design. Every single score has to--essentially--reload the engine. Have 20 channels assigned to score A? 24 channels assigned to score B? If you have A in the foreground, and click on B (even by mistake) you have to sit and wait while sounds are loaded.
Very large sound libraries (e.g. Hauptwerk, VSL) load in a separate process. You launch that application separately (it could launch with Sibelius automatically)--you use the control panel for that app, running in a separate process, to load the appropriate VSTs.
During notation, or playback, Sibelius sends the MIDI Note On/Off messages to the playback engine using a "VST Link" (strongly recommend reviewing how Hauptwerk does this--Sibelius already has a working relationship with Milan Digital Audio).
The result is that users working with multiple scores using a common range of VSTs don't have to wait for Sibelius to load/reload/crash/reboot/load sound libraries in order to work effectively.