When Solr is used in the context of a specific requirement (for example, log search for an enterprise application) it holds a specific schema that can be defined in schema.xml and copied over to nodes. The schema is based on the schema attributes indexes and thus plays a vital role in the performance of your Solr instance.
In the schema.xml file of the Solr configuration, the system allows you to specify the <defaultSearchField> parameter. This is the parameter that controls when you search without an explicit field name in your query, and which field to pick up for searching. This is an optional parameter; if this is not specified, for all the queries that are not providing the field name, the search will run them on all the available fields in the schema. This will not only consume more CPU time but on the whole, slow down the search performance.