What's you platform? If it's a VM have you checked out the guest performance in Vcenter? That gives you good insight into which resource is most constrained (CPU, memory, disk I/O, etc.).
Is the installed image the one recommended for the tier of use you are actively using? Even if so, in a VM environment you can allocate additional memory or cores to speed things up.
Other things to look for include excessive syslog message rate and lots Netflow records/second (if you're using Assurance).
