How long does the Raspberry Pi take to calculate a C coded 1024 point floating point FFT ?

Answer : 1.63 microseconds
Note :
 20 years ago (1992) the first floating point DSP with a dedicated C compiler (The Texas Instruments TMS320C30 (30 MHz)) took 16 microseconds to run the same code
 10 years ago (2002) the state of the art floating point DSP (The Texas Instruments TMS320C6701 (167 MHz)) took 0.82 microseconds to run the same code
 Today (2012) a modern Pentium laptop (2.4 GHz) can run the same code in 0.13 microseconds
Maybe I will add another comment, in the future, to compare the cost and power consumption of the various devices.

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Numerix-DSP Libraries : http://www.numerix-dsp.com/eval/