OFDM demodulation time
Once you have the various I+Q samples, you need to execute an FFT. Plus other "cleanup" steps.

The Wikipedia page states 20MegaSamples/second for 4 us, thus 80 samples. Round up to 128 samples.

If I recall rightly, the computational load for FFT is 4 * N * log2N, thus 4 * 128 * 7 ~~ 3,500 operations. At 1 nanosecond per multiple/accum/butterfly, the time is 3.5uS.

https://en.wikipedia.org/wiki/IEEE_802.11a-1999