SELECT 
  field_id 
FROM 
  cscart_profile_fields 
WHERE 
  profile_type = 'S' 
  AND field_name = 'cr_free_shipping_threshold'

Query time 0.00017

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.35"
    },
    "table": {
      "table_name": "cscart_profile_fields",
      "access_type": "ref",
      "possible_keys": [
        "field_name"
      ],
      "key": "field_name",
      "used_key_parts": [
        "field_name"
      ],
      "key_length": "98",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 1,
      "rows_produced_per_join": 0,
      "filtered": "10.00",
      "cost_info": {
        "read_cost": "0.25",
        "eval_cost": "0.01",
        "prefix_cost": "0.35",
        "data_read_per_join": "104"
      },
      "used_columns": [
        "field_id",
        "field_name",
        "profile_type"
      ],
      "attached_condition": "(`cscartdevel`.`cscart_profile_fields`.`profile_type` = 'S')"
    }
  }
}