> 4. export scheme and indexes from the original database
> 5. import scheme and indexes into my new database
> 6. copy all data from tables (not system-tables) of the original database into the new database
>
> Maybe the problem is caused by the content of system tables I did not copy into the new database?

I'd gamble on different index statistics. Backup-restore or SET
STATISTICS should help.
You could compare query plans in both databases...

SY, SD.