Topic: Notification system

Im trying to make a notification system for my app and I was thinking on the following, but I think it would slow up my app, dont know anyways.

a little example, I wan to show a notification to every user every time a new entry is posted, for example I have 100 entries a day and 100 users, so I need somewhere to store this information, so I need a notifications model

notification model

item_id (polymorphic, I want notifications not just for entries, but for events, new registrations, etc)

So this are created for every user, so for users and entries in my example will be 10.000 elements. after the user read the notification, this is destroyed.

But I dont know if this is the best idea, will it slow my app and my db?