There are many things you can tweak to optimise your system. First try raising your I/O buffer size. You will need a very fast computer to use Grosso under 256 samples, and we recommend 512 or even 1024 for good instrument behaviour.
Grosso is a complex instrument and as such can use a lot of CPU cycles. Another thing that helps is stacking multiple sections in a single instance of kontakt, which is a nice thing to try because of the colours you can get that way, and because the keyswitches in the instrument are all laid out for that.