Update Statement Is Too Slow

Hi

I have oracle database 11.2.0.4 on oracle linux 6.4

I have configure huge page on this OS .

After 5 months of my projects , when i want to update any tables their have over 20,000,000 records  , this action is too slow or if i want to alter a table to drop a column .

For example , the shahab table has 28,000,000 records , i want to execute the query in below :

UPDATE /*+ PARALLEL(20) */ SHAHAB SET FLAG0='TEST'

in Enterprise manager after sql execue i have this report :

some of statistics about my database :

show parameter sga

NAME                                 TYPE        VALUE

------------------------------------ ----------- ------------------------------

lock_sga                             boolean       FALSE

pre_page_sga                      boolean        FALSE