Since 11:54 UTC a new monitor spends its first 24 hours, or its first 50 successful checks, whichever comes first, in a learning period: the probes of each round start a full second apart instead of the usual 0.4 seconds, so a site we have never measured is never hit faster than we know it can take. The monitor page shows a line, «Learning how this site responds», while it lasts. Confirmation rounds and alerts work from the first check as always; only the spacing is slower. After the learning period the monitor gets its own pace from its measured response time, between 0.4 and 1 second, recomputed every hour, as How a check works describes. Existing monitors are not affected, and heartbeats have no learning period.
This comes from yesterday's cases: the first rounds on a site we know nothing about are the ones most likely to be too fast for it.