Database performance is slow after upgrade from 10g to 11g

I upgraded the database into 11g, in the alert log I see the warning message
WARNING: Heavy swapping observed on system in last 5 mins.
pct of memory swapped in [4.36%] pct of memory swapped out [0.31%].
Please make sure there is no memory pressure and the SGA and PGA
are configured correctly.

Can anyone provide the suggestion on this?  Thanks

SQL> show parameter sga

NAME                                 TYPE                             VALUE
------------------------------------ -------------------------------- ------------------------------
lock_sga                             boolean                          TRUE
pre_page_sga                         boolean                          FALSE
sga_max_size                         big integer                      40576M
sga_target                           big integer                      0
SQL> show parameter pga

NAME                                 TYPE                             VALUE
------------------------------------ -------------------------------- ------------------------------
pga_aggregate_target                 big integer                      20G