I hope you are well today and thank you for your question.
The option “On (slow, works with cache plugins)” saves and retrieves post view count using AJAX so it is slow but works with caching plugin. The other option doesn’t use AJAX.
There isn’t any reliability difference between them. If you are using cache plugin then use “On (slow, works with cache plugins)” option orelse use “On (fast, does not work with cache plugins)” option.