Since 11:41 UTC the probes in a confirmation round, a Check now or an alert-mode round are spaced by the site's own response time instead of a fixed 400 milliseconds: the fastest check the site answered over the last day, clamped between 400 ms and 1 second, refreshed hourly. A site that answers in 200 ms is checked as before; a site that needs a second per request now sees one request of ours at a time, and its confirmation round takes up to 15 seconds instead of 6. Nothing changes in the verdict rule.
At the same time only one confirmation round runs at a time against the same site, and at most three against the same hosting-panel server or the same group of monitors in an account (since 12:15 UTC; the first version allowed one per server, which delayed verdicts when many sites on one server failed together), so a server with many of your sites is never asked to confirm all of them in the same seconds. Scheduled rounds will get the same spacing with the next probe release; until then the response times shown for sites with a small worker pool still include the wait behind our own probes.