SQL Server - Comparing to NULL very slow

Question

I want to speed up the following query

There are two conditions in the WHERE clause (see below query for reference)

Currently, it takes about 60 seconds. However

Answer 1

Can you split this into two queries? or often causes a problem for optimizers:

if @Query is null
begin
    select * 
    from (select row_number() over( order by b.BookTitle) as RowNumber, b.*
          from Books b (nolock)
          where @Query is NULL
         ) as t1
    where t1.RowNumber between 40 and 60;
end
else
begin
    select * 
    from (select row_number() over( order by b.BookTitle) as RowNumber, b.*
          from Books b (nolock)
          where contains(b.BookTitle, @Query)
         ) as t1
    where t1.RowNumber between 40 and 60;
end