You could try a subquery to speed up performance.
SELECT crhd~arbpl INTO TABLE post1_plan
FROM crhd
INNER JOIN kbed ON crhd~objid EQ kbed~arbpl
WHERE kbed~typkz EQ '2'
AND kbed~plnum IN ( SELECT plnum FROM resb
WHERE matnr EQ marc-matnr
AND werks EQ marc-werks
AND bdter LT day_range_1
AND bdart EQ 'SB'
AND kzear NE ''
AND aufnr NE ''
).
The real solution is to create a new index on table KBED for fields TYPKZ and PLNUM.