"Downtime" is a under-defined in this context. The author seems to use the definition "won't cause errors or disconnections" while you are also considering the

"Downtime" is a under-defined in this context. The author seems to use the definition "won't cause errors or disconnections" while you are also considering the response time. Everyone cares about the former, while the latter is of more interest to heavily-loaded sites. The case you mention of a long-running query is important, so everyone needs to have some idea of their workload at the time.