SELECT FIRST 20 SKIP x .... Alternatives ?

I got a table with 21 columns (7x INTEGER; 5x FLOAT, the rest are varchars)
The table has about 40000 rows.

My Query:
SELECT FIRST 20 SKIP x * FROM TableName ORDER BY field1 ASC
There is an index on field1.
(All fields have an index. The field used in "ORDER BY" could be any field in the table. The sorting order is not definite too.)

This query is generated by a Table-Control and x increases as the user scrolls down (the Table-Control always shows 20 Entries at a time)

Some timings:
x=20 ~2,5s
x=40 ~2,5s
x=100 ~2,5s
x=140 ~2,5s
x=13680 ~ 2,6s
x=37960 ~ 2,7s

It seems that Firebird always creates a result-set for the whole query and afterwards filters out the non-needed rows, it also seems that the result-sets of the previous queries are not held in cache for faster filtering in subsequent SELECT FIRST SKIP's...

I am a firebird-noob ;D
Can someone give me a hint on how to speed it up ?

I don't think that Firebird fetches the whole resultset and then filters out non needed rows if it can use an index to perform the order by, I'd check the explain plan to see if index is used, should be something like:
Note that if you are ordering by a field in descending order and the index is ascending no index will be used, see
I don't think that Firebird has a cache to hold query results and then I don't think that it could be used to prefetch parts of a resultset.

Note: examples are based on EMPLOYEE sample database.

Note that there is an item with medium priority in the roadmap related to index usage

Thanks.

Problem solved: Stupid mistake ;D
I connected to the wrong database, the wrong database did not have an index on the important fields ;D