Efficient calculation of correlation function every $N^{\rm th}$ point
If you have a signal of length $N$ you can calculate auto correlation function of length $K=2N-1$.You'll need perform $N$ multiplications for each value of correlation function $r_{xx}$ or in other words you need one correlator. So you need as many correlators as your $r_{xx}$ function length is. If you want to find correlation in the lag $0$ use one correlator. You can can calculate correlation of any lag $i \in (-N+1...N-1)$ via one correlator with signal properly shifted by $i$ times. From the other side $FFT$ method allows you to find $N$ taps of cyclic auto correlation function via only one $FFT$ operation. So it is not optimal if you need only a few correlation values.