You need to explicitly populate all combinations of reduction field values that you want each end-user to see. Let say you have 3 Categories A1,A2,A3 under PRODUCT A. Your Section Access table (for Qlik Sense) should be:
Load * inline
ACCESS ,USERID ,PRODUCTID,CATEGORIES, OMIT
USER, UserA, PRODUCT A, A1,
USER, UserB, PRODUCT A, A1,
USER, UserB, PRODUCT A, A2,
USER, UserB, PRODUCT A, A3,
Section Access table is no special than other tables in your data model except for the fact that it is hidden from users. If your reload takes long time, Section Access isn't likely to be the cause.
If you still get error, some screencaps of the error messages should be helpful.