Top 10 priorities in performance optimization of a complex Drupal site
I won't talk about APC and other Apache optimization tricks. 

In in experience:


Enable caching for Views and reduce use of Views
Enable Javascript and CSS compression
Minimize the modules. Disable unused ones. In production I disable as UI's .. the 
following script will give you some help.
Most of bottleneck comes from Mysql. I suggest you optimize it and I found this post useful.


Disable User Interface Modules

When not developing you can safely disable Field UI, Rules UI, Views UI, dblog etc. Also I'm sure but we should also be able to disable CKEditor/FCKEditors.

This is what I use in a script:

drush dis --yes admin_menu advanced_help backup_migrate content_type_overview  dashboard diff environment_indicator field_ui help module_filter rules_ui search term_merge update views_bulk_operations views_ui vppr  field_ui  rules_admin devel site_audit  drupalgeddon l10n_client taxonomy_manager dblog