By default, Piwik reports are processed when you request them in the User Interface. Please see How to setup auto archiving of your reports every night? to automate the generation of Piwik reports. See also How long does the Piwik archiving process? for expected speed for high traffic websites.
If you are running Piwik analytics for a high traffic websites (or hundreds of them), see our guide Optimize Piwik server for speed.