row_number performing badly in table with a lot of rows

experimenting with using row_number to implement a SKIP type function.
Where an inner query use row_number to assign a sequential number to the
selected rows. And then the outer query says WHERE rownum >= 20 to skip
over the first 20 of the selected rows. The objective being to page thru
the rows of a table, either forward or backward.

Here is the code for paging forward:
exec sql
select a.seqnbr, a.rownum
into :vSeqnbr, :vRownum
from (

select a.seqnbr, row_number( )
over( order by a.seqnbr ) rownum
from dsh3526p a
where a.seqnbr > :inTopKey.seqnbr
order by a.seqnbr
fetch first 20 rows only ) a

where a.rownum >= :inSflPagSx
order by a.rownum
fetch first row only ;

The code works. And it performs well enough when there are up to 50,000
rows in the table. But the more rows in the table the slower the execution.

How can I use row_number in this manner efficiently? I tried the code
where I have a 3rd nested query which simply runs "fetch first 20 rows
only", returning those rows to the query which assigns the row_number which
returns its result rows to the outer query. Same poor performance.

thanks,