Welcome back!

It all depends on what you do with it. I have never touched an N9 so I can't say, but I'd never say the N900 is or feels slow. YMMV, obviously.

While you were away, freemangordon started the cssu-thumb project, where many programs are being recompiled using gcc 4.7.x and the thumb2 instruction set (which now runs stable thanks to the linker in >= 4.6.x plus kernel-power >= 51r1).

I have not personally installed cssu-thumb because of the cssu part (which still doesn't quite convince me), but hearing the reports of people who have tried it makes me think that I should try it at least on one of my 3 N900s

The idea with thumb2 is that the programs take less space, which in itself is a blessing for the N900's limited rootfs. But less space also means that the code segments take less space in memory, meaning a bit less RAM is used, and *that* is a blessing for the N900 (obviously most RAM is taken up by dynamically allocated blocks, which will take the same space regardless of the instruction set).

Since AFAIR you were already using CSSU on your N900 (hope you still have it), you should be OK with it.

Anyway, even if you opt for the N9, enjoy it