Is higher RAM frequency or capacity better?

Question

RAM

Synology has a 2 GB 1866 MHz RAM stick.
What is better to buy DDR3L 8 GB 1600 MHz or 4 GB 1866 MHz. They cost about the same. In one case, I will not get so much RAM, but its frequency will remain high, in the other, there is significantly more capacity, but the frequency of all RAM will drop.

Answer 1

is it a NAS?
Did you miss 2GB? how did it determine that you expect to speed up by adding 4 or 8 GB? Do your tasks fit well into the small cache?
For consumer NAS tasks, there is no difference what frequency the memory has, since it is not fully utilized in almost all typical user tasks (to load the processor, you need many simultaneous small requests, for example, to load an atypical task, such as a web server and cause a habraeffect, but even here the difference between 1600 and 1866 frequencies of the RAM will not be noticed even by benchmarks).
So the answer is more memory is better than less.