Sam H. wrote in a MDL-26774 comment:
"There are alot of database queries being made within the users foreach loop - I'm not too sure what the priority on this issue is but certainly if this doesn't need to get in as soon as possible you could GREATLY improve the performance of this webservice by absrtacting the database queries and performing them in bulk... perhaps should be looked into anyway... it may pay to talk to Martin about it.
Getting the users enrolled courses is one area that could be greatly improved.
Users shared enrolments is another area."

Most likely the different core function should be made bulk-enabled, probably looking if user id is an array or not...

Define what exactly need to be improved (custom fields, list of enrolled courses, preferences...) and how to do it (core function should support bulk user id, request needs to be rewritten...)