Is there a module provides UI to create field index? My views are quite slow in some cases, needs create indexes on field tables. Is there a module for that?
Because views does not create tables but rather builds queries, you cannot add indexes to views but what you can do is insert Mysql Index hints via the Views Index Hint module.

If this module is not flexible enough for you, you can manually add the index hint after the sql query is constructed by using the hook_views_query_alter hook.