How to reduce frequency resolution for high sampling rate and lot of samples?
A simple way is to split the data into smaller N sample segments (pick N to suit your desired frequency resolution, then average the power or amplitude of all the FFT spectra:

$$
  \hat X = \frac{1}{N} \sum^{N-1}_{n=0} X_n  X_n^* 
$$

Where $X_n, n \in \left\{0:N-1\right\}$ is your series of shorter FFT results and $\hat X$ is the averaged power spectra.

If I remember correctly, for noise signals, the variance is reduced by $\frac{1}{N}$.