First, you should probably eager load the comments, like so:

$posts = Post::with('comments')->where('category_id', '=', 5)->get();

This bit doesn't do anything really:

$posts->each(function($post) {
      $post->comments;
});

And this bit shouldn't work at all if there are comments, cause you can't really print a collection.

{{ $post->comments }}

As for your question about ordering your comments, you could do something like this:

$posts = Post::with(['comments' => function($query)
{
    $query->orderBy('commentTimestamp', 'desc');

}])->where('category_id', '=', 5)->get();

Again, comments are being eager loaded, but then we order them as well at the same time.

In fact, I am aware of the eager loading solution.
But I want to evaluate posts and comments separately for caching. This way, if $comments changes for a post, I will only query $comments of this post, not all $posts and their comments ($posts are up to date).

Depending on how many posts you have per page eager loading might still perform better than caching comments.

Eager loading does 2 queries.

The first time you do that while caching the comments you have 1 query to get the posts and then 1 query per post to get the comments. Not very scalable. Any further views, though, you'll then only do 1 query.

You could just cache posts including the comments and then invalidate the cache when a new comment has been added. This should work fine unless you have an extremely busy site where people comment a lot. Gives you 2 queries initially, then 0 afterwards.