Sailfish is partly closed source.

Nemo works on the N900 and should be compatible with Sailfish.

Sailfish looks pretty fast on an N950 -- which has roughly identical hardware to the N9.
Unfortunately for N9 owners, their hardware is quite closed. By comparison, the N900 is more open but very memory limited. It's a better bet to get Sailfish apps running satisfactorily on the N900 than on the N9 but it depends how lean Nemo becomes.