Why queries takes too much time to execute?

    Hi.

    I´m new here and i´m not sure if this is the right email to solve my problem.

    Well, i have a very large database, with vary tables and very registers. Every day, too many operations are perfomed in that DB, with queries that insert, delete and update. Once a week some statistics are collected using vacuum analyze.
    
    The problem is after a period of time (one month, i think), the queries takes too much time to perform. A simple update can take 10 seconds or more to perform.

    If i make a backup, drop and recreate the DB, everything goes back normal.

    Could anyone give me any guidance?