Pre K1 1.1.14 a search for users would not match across first and last name fields, making it impossible to find a user by searching with the conventional name. E.g. "John Smith" would not find any one.
KNL-691 this was changed so that the search entry is broken into tokens on spaces and matches are now based on an OR of finding any substring of any token in the search string in the email or name fields. Based on a code review it appears that the underlying query uses wildcards (see DbUserService.java search) and an UPPER function (see UserServiceSqlDefault.java). The upper prevents the use of an index. This means that query is potentially very expenseive when given multiple terms on a large set of users.
This should not be wide spread problem since the query is only run by the admin user.