Every night, if you setup auto-archiving, Piwik will run through the logs and compile all your reports for each day, week, month and year. How long the process will take depends on your traffic, number of visits and pages, on the number of websites you have setup in Piwik… and of course on your server performance.
For example, using version 1.3 on a single core duo2 server with 3GB of ram, Piwik archiving process takes about 2 hours for 400,000 pages per day (and more than 6,000 websites). We are constantly improving Piwik performance: making Piwik faster is a top priority. Stay tuned… or come and help us if you know how to design software to make it faster.
Also, if you have a medium or high traffic website, we would be interested to hear your experience with Piwik archiving. How long does archiving take for your traffic, how many visitors/pages, how many websites, what type of server; Please post your numbers in the High traffic Piwik servers forum post, and let us know by email.
For practical information about managing Piwik for high traffic websites, see this FAQ: Piwik web analytics for high traffic websites check list.