Was this the template? That's very much a worst-case scenario, which is why the file says at the top you shouldn't try to play it all at once. Basically, Sibelius is not very efficient when it's sending data to the VST. The template is sending a lot of data to the VST (keyswitches, etc), but doing it for every instrument at the same time. No real score would ever do this, of course.
This Sibelius issue isn't something I can do anything about, but it's definitely true that with real music, even the whole orchestra can be in use with not that much crackling (though you would be advised on a 64Gb machine to up the prefetch to 32768, which results in a 21Gb memory footprint). Zoids for instance should be a lot smoother.
There's another couple of gotchas to watch for: first of all, when the file opens, the Spitfire plugin will still be loading the samples - Spitfire's manual advises you to wait until they're loaded, which can be seen from a flashing light in the corner if you've got an instance open.
Second, I've found that if you have the library loaded and you leave it for any length of time, Windows starts "tidying up" the memory: you can see if it's doing this because task manager will show the Sibelius process using a lot less memory than it should. Sometimes this results in disastrous, horrible consequences.
In summary I would: (1) reboot Sibelius, (2) load Zoids, (3) change the prefetch in Audio/Settings in Spitfire itself to avoid note steal, and (4) wait until the samples are loaded before pressing play.
I really need people not to play the template all at once though. Maybe I should make the warning a bit larger...