How many seconds does it take for each screenshot to be produced?

The length of time taken for each screenshot is depends on a lot of factors. Here are some of them:
- the length of time it takes the guest configuration to boot.
- the reponsiveness of the site.
- the length of the web page matters, as the full page screenshot has to scroll down, take a photo, then scroll down some more. It continues this until the entire page has been captured, then stitches together the images.
From an accounting standpoint, you are just charged for the time after the configuration is up and starting the screenshot script - not the time for the guest os to boot.