Oracle Database Discussions

Hi,

I have a table:

MyTable (
f1 varchar2(4),
f2 varchar2(2),
f3 varchar2(10),
f4 varchar2(4),
f5 number(2),
f6 date,
f7 varchar(2)
PrimaryKey (f1,f2))

the table has 1 b-tree and 2 bitmap inserts.
I receive enq TX - row lock contention during insert

insert into MyTable (f1,f2,f6) values (:a,:b,sysdate);

and the insertion is too slow. The only wait observed on the database is row lock contention which has a huge occurrence (more than 512msec waits). Can you help me with this problem. How does the inserts affect row locks and what is the reason for waits?
regards