It has been documented that custom fields with global context can become a performance issue for JIRA, so the recommendation has been to limit each field just to the projects in which it is relevant.

As a Jira admin, it is difficult to manage the project contexts of a field in the current interface (as of version 7.10.2 at least).  The multi-select list requires that you hold down CTRL when clicking a project in order to add it to the existing list rather than replacing the existing selections, and you can only see a few projects at a time so there is a lot of scrolling involved.

Please consider changing this to be like the interface when adding issue types to an issue type scheme – two boxes, left and right, and you move items from box to box in order to include or remove them.