What would cause a commit of an update be incredibly slow??

We have a simple table with a primary key PKID (number), foreign key to a parent table FKID (number), and COMMENT varchar2(2000) field with a direct store text index on it.  Lately, an update to this table to set COMMENT to a new value where FKID=nnnn followed by a commit is taking as long as 20 seconds for most users, while for others it is taking a second or two.  All users access the database through the same middleware.  On the database server, I connected as a regular user and ran a typical update myself:

SQL> update thistable set COMMENT='updated comment' where FKID=nnnnn;