User feed in DB (possibly DynamoDb)

I am thinking to use DynamoDB for user feed generation. I will store UserId (Hash) with PostId (Range). But I need to keep only last 3000 posts in the DB. So I am thinking about having a background task that will clean up the table. Is this something reasonable to do ? I'm not sure if this type of range queries will work reasonably fast, since I have about 25mln user records.

Please suggest any other options (except fanout in redis) that might work.