DAX Optimizing SUMMARIZE()

I'd recommend bringing the minimum calculation outside of the FILTER iterator if possible. Maybe like this:

app_days_per_user =
VAR User_days =
    ADDCOLUMNS (
        SUMMARIZE ( 'Event Detail', 'Event Detail'[user_id] ),
        "app_days",
            CALCULATE (
                VAR MinStart =
                    CALCULATE ( MIN ( 'SecDB Sites'[EffSubStartDate] ) )
                RETURN
                    CALCULATE (
                        DISTINCTCOUNT ( 'Event Detail'[day_as_timestamp] ),
                        FILTER (
                            VALUES ( 'Event Detail'[day_as_timestamp] ),
                            'Event Detail'[day_as_timestamp] > MinStart
                        )
                    )
            )
    )
RETURN
    AVERAGEX ( User_days, [app_days] )