You can very well join GLPCA and MARA via MATNR, however then you need WHERE-conditions that effectively reduce the GLPCA entries.
You have a bunch of selection ranges that might contain anything/nothing at runtime, so the response time is quite unpredictable by just looking at your code.
You should study the available indexes for GLPCA and see if you can add WHERE-conditions that make effective use of one of these indexes. Also look into technically forcing the user to enter narrow selection criteria before executing the report.
Thomas