Unable to account for application waits against session history and mined transactions

We have an application that performs lots of small transactions frequently.  Sometimes, seperate sessions attempt to lock the same row and in some cases, we are seeing waits of around six seconds for transactions which one would expect to be very transient.

I am attempting to account for these occasional anomolies.  Here is a section of the v$active_session_history that I recorded:

                                                                                                                       Wait                                                                            
Sample         Wait  Block SQL            Object                    Wait                                               Time
time            SID    SID ID                 ID File    Block  Row Event                                            (usec)
------------ ------ ------ ------------- ------- ---- -------- ---- --------------------------------------------- ---------
11:02:21.247    879        7zsrt2k0ykx3t  133821   21    50690    0 db file sequential read                            1540