The kernel UserAndRoleDirectoryServiceImpl.java cache is hard-coded for 1 minute expiry time with maximum size of 200 users:
private final LoadingCache<Tuple<String, String>, Object> cache = CacheBuilder.newBuilder()
This is OK for small installations, but probably sub-optimal for larger sites with many users (e.g. accessing Opencast through LTI or external auth mechanisms), especially on the admin/presentation nodes.
Another reason for developers to set a longer cache time on local builds is that so cache invalidation errors (e.g. when a user or group is updated) are more noticeable.