SQL - improve NOT EXISTS query performance

Question

Is there a way I can improve this kind of SQL query performance:

INSERT
INTO ...
WHERE NOT EXISTS(Validation...)

The problem is when I have ma

Answer 1

Outer Apply tends to work for me...

instead of:

from t1
where not exists (select 1 from t2 where t1.something=t2.something)

I'll use:

from t1
outer apply (
    select top 1 1 as found from t2 where t1.something=t2.something
) t2f
where t2f.found is null