Is this operation slow?

I am going to implement voting for posts. There are considered two variants:

  1. For each post the user can vote once.

  2. User has N (e.g. 15) votes per day and can vote from for up to 15 posts to for one post 15 times.

In the variant 2, I need to show user’s votes for each post, therefore read the DB for each displayed post (in a post stream similar to Twitter), with key combining user ID and post ID.

So, may the variant 2 tend to be too slow or to costly?

I suppose with enough activity that it will become expensive, either in computation, or in storing a bunch of indexes. But to find out how bad it really is you only can build it and measure it in practice