My queries are slow under load
I know it's late but there is a lot that could be going on.

-Make sure your statistics look good:

select owner,table_name,sample_size,num_rows,last_analyzed
from dba_tab_statistics.


num_rows and last_analyzed should match if you use default settings.

select owner,table_name,index_name,sample_size,num_rows,last_analyzed
from dba_ind_statistics.


Again sample_size and num_rows should match.


Verify if you use CURSOR_SHARING=EXACT or FORCE (if SIMILAR, change it to FORCE),(will require sysdba privilege):

SQL>show parameter cursor;
SQL>alter system set cursor_sharing=FORCE;
Note that ideally you want to match the parameters you have on your production system. You may want to change these to test them as a cause of your problem however.
Verify that your SGA is sized properly:

select * from v$sga_target_advice
You would get something like this:

SGA_SIZE SGA_SIZE_FACTOR ESTD_DB_TIME ESTD_DB_TIME_FACTOR ESTD_PHYSICAL_READS



 10240             .25    131401926              1.2592          1.4876E+10
 15360            .375    117178544              1.1229          1.1815E+10
 20480              .5    112472201              1.0778          1.0648E+10
 25600            .625    109873799              1.0529          1.0250E+10
 30720             .75    107077125              1.0261          9638685159
 35840            .875    105929237              1.0151          9404059270
 40960               1    104353499                   1          9058914623
 46080           1.125    103706507               .9938          8926654470
 51200            1.25    102788197                .985          8724640673
 56320           1.375    102391653               .9812          8646734008
 61440             1.5    101807274               .9756          8515379746
 66560           1.625    101275077               .9705          8394896181
 71680            1.75     97987941                .939          7696453864
 76800           1.875     97967070               .9388          7696453864
 81920               2     97967070               .9388          7696453864



Where SGA_SIZE_FACTOR is 1, is my current SGA_SIZE.
You can look at column ESTD_DB_TIME_FACTOR and ESTD_PHYSICAL_READS to see how much benefit I get if I increase it to the sizes mentioned in SGA_SIZE. In my case, not much benefit which means I'm good.


Compare your query when executed with literal values vs bind variables.
You may want to see if you have a lot of hard parses for a given query. Check v$sql and v$sql_plan (have a look at column child_number for a given sql_id.
Do you have very skewed data on some of the columns in your where clause, joins and group by? If so you may be experiencing issues related to skewed data if bind variables are used.