Subject Delete and performance afterwards

In a database with containing only one table (details see below) I
insert first around 20000 records and afterwards I delete half of
them. After deleting this records I have performance problems
accessing the table. It is best shown when I do a GBAK:

GBAK-Time after inserting before deleting: 6min
GBAK-Time after deleting: 25min

During GBAK of the deleted version I see Interbase using a lot of CPU
time but not accessing hard disk very often. This is different to the
non deleted version which is doing mainly I/O stuff. The additional
time for the deleted version is used when GBAK writes the table with
the deleted records.

I'm using IB 6.01 on W2k (Standard installation without any change on
probably performance sensitive parameters).

My database has the following structure:

create table Test (
TestID Numeric(18,0) not null,
Code SmallInt,
Text VarChar(120),
constraint Test_PK Primary Key (TestID));

create index test_idx on test(code);

create generator IDGen;

There are no other tables in the database. The database is filled with
random data for field Text (length between 10 and 40 characters) and
random data for field code with values between 0..3.

My delete is: DELETE FROM Test WHERE Code in (1,3)

Has anybody an idea about this behavior?

Thank's for any help and hints!

Daniel Achermann