Have you checked out the •••ReGranulizer••• instrument? It does do that 
Generally, I haven’t found native granular techniques to be very cpu intensive, at least not the way I’ve done it with sxx commands in phrases. I think the per instrument limit of 12 simultaneous samples caps the cpu demand nicely.
In any case, it’s good fun to push the boundaries of what’s capable within renoise. Cheers. 