How long have you been practicing/exercising the Sarod movement? It probably just takes more time.
Also, 10 notes / second is what, sixteenths at 150 bpm? I'd bet that you could improve upon this with an anchored technique if you wanted to. Again, it probably just takes more time.
But to answer your question, yes, I'm on the Sarod learning curve too and I can't keep the flutter going forever. I've actually been keeping track of my progress over the last few months though and I'm seeing improvement. I find it harder to keep the flutter going on a single string than I do when switching to adjacent strings. I think this is because for me it's easier to keep it going with a bigger wrist rotation. So I work on both.
When I 'just play' though, I still fall back to scalpal and wrist picking.