On the course's Participants page, when manually enrolling users
the AJAX call might get excesively slow, due to the treatment
of the perpage parameter as a string, which gets concatenated with the
number 1 (perpage: parseInt(perpage) + 1) instead of performing a
numerical addition.
If there are for example 1500+ users on the site, the method
core_enrol_get_potential_users will be executed with a value of 15001
for the perpage parameter, rendering painfully slow.