Hi ,
Please get help from ABAPer to investigate whether any user-exit is in place to calcuate some tax "CLASS=ZCL_IM__GTA_TAX_CODE\METHOD=IF_EX_FI_ITEMS_CH_DATA~"values before the results are displayed. If yes, that section needs to be fine tuned.
Secondly when you execute the program monitor which select statement and on which table more time is being spent. You may use SM50 or ST04 - > Oracle session for selected PID.
Hope this helps.
Regards,
Deepak Kori