The performance with IN and NOT IN can be a little tricky, because then initially loads all the data within the sub query to the memory before it matches the records. So if there are more rows, it'll drain the performance. So try with EXISTS and NOT EXISTS instead, like this

select *
from ALERTTRAN_01
where PARENT_ALERT_ID not EXISTS (
    select PARENT_ALERT_ID
    from nonbulkclosealert WHERE PARENT_ALERT_ID = ALERTTRAN_01.PARENT_ALERT_ID 
)

Here are a few helpful links that'll explain the advantages of EXISTS over IN

https://www.sqlservercentral.com/blogs/not-exists-vs-not-in

https://www.red-gate.com/hub/product-learning/sql-prompt/consider-using-not-exists-instead-not-subquery

The performance with IN and NOT IN can be a little tricky, because then initially loads all the data within the sub query to the memory before it matches the records. So if there are more rows, it'll drain the performance. So try with EXISTS and NOT EXISTS instead, like this

select *
from ALERTTRAN_01
where not EXISTS (
    select PARENT_ALERT_ID
    from nonbulkclosealert WHERE PARENT_ALERT_ID = ALERTTRAN_01.PARENT_ALERT_ID 
)

Here are a few helpful links that'll explain the advantages of EXISTS over IN

https://www.sqlservercentral.com/blogs/not-exists-vs-not-in

https://www.red-gate.com/hub/product-learning/sql-prompt/consider-using-not-exists-instead-not-subquery

The Performance with IN and NOT IN can be a little tricky, because then initially loads all the data within the sub query to the memory before it matches the records. So if there are more rows, it'll drain the performance. So try with EXISTS and NOT EXISTS instaed, Like this

select *
from ALERTTRAN_01
where PARENT_ALERT_ID not EXISTS (
    select PARENT_ALERT_ID
    from nonbulkclosealert WHERE PARENT_ALERT_ID = ALERTTRAN_01.PARENT_ALERT_ID 
)

here are a few helpful linkS that'll explain the advantages of EXISTS over IN

https://www.sqlservercentral.com/blogs/not-exists-vs-not-in

https://www.red-gate.com/hub/product-learning/sql-prompt/consider-using-not-exists-instead-not-subquery

The performance with IN and NOT IN can be a little tricky, because then initially loads all the data within the sub query to the memory before it matches the records. So if there are more rows, it'll drain the performance. So try with EXISTS and NOT EXISTS instead, like this

select *
from ALERTTRAN_01
where PARENT_ALERT_ID not EXISTS (
    select PARENT_ALERT_ID
    from nonbulkclosealert WHERE PARENT_ALERT_ID = ALERTTRAN_01.PARENT_ALERT_ID 
)

Here are a few helpful links that'll explain the advantages of EXISTS over IN

https://www.sqlservercentral.com/blogs/not-exists-vs-not-in

https://www.red-gate.com/hub/product-learning/sql-prompt/consider-using-not-exists-instead-not-subquery