Sorry, but there is no easy way to cancel this. Best to just wait a bit.
It took around 30 seconds to display this page when I timed it using Firefox on my "1.4GHz Toshiba Satelite Celeron M Processor" laptop (screen short-circuit, alligator clip) running Windows XP (IE7 was around 20% slower).
In IE browsers, you may have to click through an "it's taking too long, do you want to abort" prompt.
There are around 50 charts in the GChart paint test, all on one big page (for performance reasons, most GChart applications should limit themselves to a few charts per page).
This test page used up around 50MB in Firefox and around 20% less in IE7.
A paint test means, we display a bunch of charts and visually inspect them to see if they look as expected. The test charts each have a footnote that describes what the chart must look like if it is to pass the test.