Do you have Omnisphere 2.6 ?.
Its free for all registred Omnisphere 2 owner.
I think, if i recall it, when i did install
Omnisphere 2.5 , Omnisphere became really slow,
sluggish, even loading sound.
After installing 2.6, Omnisphere became fast,
no more problems.
I have Omnisphere 2.6 installed on a ssd.