Fastest check if row exists in PostgreSQL

How about simply:

select 1 from tbl where userid = 123 limit 1;

where 123 is the userid of the batch that you're about to insert.

The above query will return either an empty set or a single row, depending on whether there are records with the given userid.

If this turns out to be too slow, you could look into creating an index on tbl.userid.

if even a single row from batch exists in table, in that case I don't have to insert my rows because I know for sure they all were inserted.

For this to remain true even if your program gets interrupted mid-batch, I'd recommend that you make sure you manage database transactions appropriately (i.e. that the entire batch gets inserted within a single transaction).

Use the EXISTS key word for TRUE / FALSE return:

select exists(select 1 from contact where id=12)