I own a single site on M4.2 and after updating to from 4.1.4 to 4.2 I saw a constant uptime of 0.30/0.40, even without user activities on the moodle hosted on this server, which usually scores 0.02/0.08 ...
Now that i set up the cron keepalive time from 3 minutes to zero, the uptime has returned to "normal" values.
I'm managing this 4.2 instance for experiencing the new features (access is restricted to Italian Moodle User Association members), so I'm not so concerned about performances and I didn't realize it until I read this thread.
You are right in your description of the problem: probably in low-performance hosting this parameter has to be managed to balance between responsiveness and cpu load.