Performance: don’t match related posts when enough manual posts are present

Currently, plugin tries to find related posts by matching content and then merges them with manual posts. So if we want 4 related posts and we already set 4 manual posts, plugin will still try to find matching posts with its algorithm.
I suggest reversing the order: first we get manual posts and if there is not enough of them, fill the rest with matching posts.

I know that plugin already has post and HTML cache, but there is still initial performance cost.