How to reduce idle time of a Postgres connection using SQLAlchemy?

I have an app built using Fastapi & SQLAlchemy for handling all the DB-related stuff.

When the APIs are triggered via the frontend, I see that the connections are opened & they remain in IDLE state for a while. Is it possible to reduce the IDLE time via sqlalchemy?

I do the following to connect to the Postgres DB:

import sqlalchemy as db

eng = db.create_engine(<SQLALCHEMY_DATABASE_URI>)
conn = eng.connect()
metadata = db.MetaData()
table = db.Table(
            <table_name>,
            metadata,
            autoload=True,
            autoload_with=eng)
user_id = 1
try:
    if ids_by_user is None:
        query = db.select([table.columns.created_at]).where(
                table.columns.user_id == user_id,
            ).order_by(
                table.columns.created_at.desc()
            )
        result = conn.execute(query).fetchmany(1)
        
        time = result[0][0]
        time_filtering_query = db.select([table]).where(
            table.columns.created_at == time
        )
        time_result = conn.execute(time_filtering_query).fetchall()
        conn.close()
        return time_result
    else:
        output_by_id = []
        for i in ids_by_user:
            query = db.select([table]).where(
                db.and_(
                    table.columns.id == i,
                    table.columns.user_id == user_id
                )
            )
            result = conn.execute(query).fetchall()
            output_by_id.append(result)

        output_by_id = [output_by_id[j][0]
                        for j in range(len(output_by_id))
                        if output_by_id[j]]
        conn.close()
        return output_by_id
finally:
    eng.dispose()

Even after logging out of the app, the connections are still active & in idle state for a while and don't close immediately.

Edit 1

I tried using NullPool & the connections are still idle & in ROLLBACK, which is the same as when didn't use NullPool