When switching from a small site report to the All Websites pages, it's possible to make Piwik overload and unresponsive. We should have some kind of "stop all queries" / "panic button" available.
Use case : On a small site I had a Date range Jan 2014-November 2015, 10k visits but once done with it clicked on "All Websites". Piwik keeps the Date Range (that's expected) but with 80 millions lines in my Piwik DB and just 15 sites, Piwik became unresponsive. I closed the page and login back but it took 3 minutes to finally have something showed (I login to "today" by default).
This would be ideal for a new plugin. Maybe someone from the community is keen to work on it and to publish it on the Marketplace?