Throttle background parsing by about 40% to reduce overall resource usage.
Log how long it takes to parse each container.

git-svn-id: https://plugins.svn.wordpress.org/broken-link-checker/trunk@880835 b8457f37-d9ea-0310-8a92-e5e31aec5664
Minor: fix typo.

git-svn-id: https://plugins.svn.wordpress.org/broken-link-checker/trunk@880836 b8457f37-d9ea-0310-8a92-e5e31aec5664