Depending on your influence on the source file you may try to build it with bulk inserts instead of single row inserts:
insert into tbl_a (x,y) values (1,2), (4,5),...,(33,2);
in addition to this you can disable index maintenance for non unique keys for the affected tables first by
alter table tbl_a disable keys;
and afterwards
alter table tbl_a enable keys;
Hope this helps.
Andreas