The current completion cron has three parts.
The first two take less than a minute (usually) to run, this is what manages the completions themselves.

The third part is the mark aspect. This marks users onto a course by checking the enrollment types. This process is very slow and can take up to one hour, this can cause sites to slow down or even stop functioning all together if there is more than one site on the same server.

The proposed solution to this would be to split the completion cron out into two crons one recommended to run daily (the slow one) and one to run every minute (the fast one).

This ensures that the completion cron can mark completions within a minute without causing sites slowdown.

This issue affects previous versions of Moodle but we are looking to improve this for Moodle 3.0.

Once fixed we would expect the completions to update minutely without issue and for the users to still be enrolled once daily.