Why can FFT of full vector measure the irregular access performance in MATLAB benchmark?

Hello Everyone,

I read the discription of MATLAB built-in benchmark, which execute six tasks and evaluate the performance of PC.https://de.mathworks.com/help/matlab/ref/bench.html

One of these tasks is: FFT of long vectors and for this task perfomance factor is floating-point and irregular memory access. What is irregular memory access and why can FFT measure it?

Thanks!