Hi,
time consuming is because may be your running it for hole year bseg is huge table.
if you want to copy lifnr also in similar way change the MODIFY statement like below
MODIFY gt_bseg FROM ls_bseg TRANSPORTING kunnr lifnrWHERE belnr = ls_bseg-belnr
AND bukrs = ls_bseg-bukrs
AND gjahr = ls_bseg-gjahr.
if you dont want to display the data then comment below lines
* DATA GR_TABLE TYPE REF TO CL_SALV_TABLE.
*cl_salv_table=>factory(
* IMPORTING
* r_salv_table = gr_table
* CHANGING
* t_table = gt_bseg ).
* gr_table->display( ).
Regards,
Pavan