ElasticSearch Performance optimization

Question

I have a single node dedicated for ES server. We have indexed around 25 GB Data in it.

I am using a bool query to fetch data and it takes around 6-7 minutes to give the results.

The RAM on the node is just 1 GB. I understand that the RAM and other hardware parameters needs to be increased.

But apart from hardware improvements, is there any thing else that I can do?

Updated: Below is the mapping of my index: Below is the mapping of my index:

:index=>index_name,
        :body=>{
          :mappings=>{
            :texts=>{
              :properties=>{
                :file_number=>{:type=>"long"},
                :text=>{:type=>"string",:term_vector=>"with_positions_offsets_payloads"}
              }
            }
          }
        }

Below is the query used :

:body=>{
        :query=>{
        :bool=>{:minimum_should_match=>1,
        :should=>[{:query_string=>{:query=>"master agreement", :default_field=>"text"}}
                 ,{:query_string=>{:query=>"master agreement", :boost=>2, :default_field=>"text", :default_operator=>"AND"}}
                 ]
               }
                },
         :fields=>["file_number"], :size=>393413, :from=>0
       }

I am not using pagination. I am scanning across the full set

Please let me know