Hi Arnet

It does sound like a cron setting for the Learning Plan assignment is causing this delay. To test this out you can usually run the cron manually by typing in yoursitename/admin/cron.php

It is possible to adjust the frequency of these . I checked in the latest version 2.7 which has a Scheduled tasks that lets you adjust these and displays  the frequency. The only setting around Learning Plans is process plan autocompletion with the default of running ASAP so not sure if you the learning plan assignment can be made more frequent.

I think there is some documentation on the cron frequency for earlier versions but I haven't been able to locate this

There are some instructions on changing the frequency on http://help.totaralms.com/totara_v2.6_help/Cron.htm

regards