Hello
Why would you not believe it runs at 16MHz?
Simply because I didn't read it on the datasheet... but I guess I can trust Massimo! 
If you are using an original arduino you can see by yourself by looking at the quartz next to the processor.
It should say S16B00K7 the 16 means 16MHz
Nice!
in terms of floating point speed it all depends on the quality of the software implementation as the processor doesn't have a floating point coprocessor
we are using the one that comes with avr-gcc so you should check if there is any indication on the avr-libc website
The question is... how may cycles might it take to sum or multiply two floats?
I've found a few documents (page 34 of http://manet.die.udec.cl/~biomedica/senales/avr-libc-user-manual-1.6.1.pdf and avr-libc: <math.h>: Mathematics) but they're not useful for my purpose.
...Any idea?
PS: I thought I might calculate how much time it takes for two floats to be added or multiplied but I need a more precise version of the millis() function... does a version of millis() that work with microseconds (instead of milliseconds) exist?
Thank you.