By default, this screen uses W3C's HTML validation service. This is a donation-funded service.
Scrutiny currently starts validation as soon as the first page is crawled. By default it will validate the first url only but you can choose to validate all of your pages via Preferences. Scrutiny respects W3C's wish for automated requests to be no less than a second apart. This is why validation may still be running after your crawl has finished.
The public service is not always available and has limits, reporting 'The Request Timed Out' after a certain number of checks.Consider running your own instance of the validator.
It's possible to download, install and run the validator on your Mac for free. You can use your own instance to validate all of your pages with a shorter delay between pages.
I can't support you with this, but if you are successful, you can enter the url of your instance of the validator in the appropriate box in Preferences.
As an alternative to the installation instructions above I can recommend the free 'Validator S.A.C' from Chuck Houpt. It's easy to download and run. You will need to start the validator as a web service but this is relatively easy too and the instructions are on the same page as the download. You will then need to enter http://localhost/w3c-validator as the location of the validator.