Clause in your UDF appian.dbo.udf_A

"WHERE ... datediff(day, [login_log].[dt_login], Convert([@date]))<=[@last_x_day"

eliminates all indexes and makes server scan whole table and return all rows, because it cannot predict result of the calculation.

Change it to

[login_log].[dt_login]>= datediff(day, - @last_x_day, Convert([@date]))

and make sure you have clustered index on column dt_login.