Why does the footage look so grainy when I shoot in super slow motion?

I am only using the 6 sec record and it looks like the shutter speed is around 1500 or 2000.

(It will probably look better if I shoot it in 60i and slow it down in Final Cut, but that defeats the purpose of that feature on the Z5u)

Any thoughts?

Thanks.

Because the picture quality in slow motion modes on the Z5/FX1000 is compromised, and the longer you buffer, the worse quality it gets.

I've found in good light, the 3 second buffer is good enough to get away with it in most cases, but the longer modes are too poor to be used for anything other than the odd special effect or web video where the quality doesn't matter too much.

In low light, it gets too grainy to be used much...