SELECT 
  value 
from 
  cscart_profile_fields_data 
WHERE 
  `object_id` = 70548 
  AND `field_id` = 56

Query time 0.00036

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "2.24"
    },
    "table": {
      "table_name": "cscart_profile_fields_data",
      "access_type": "ref",
      "possible_keys": [
        "idx_entire_row",
        "idx_field_object",
        "idx_obj_objtyp"
      ],
      "key": "idx_field_object",
      "used_key_parts": [
        "field_id",
        "object_id"
      ],
      "key_length": "6",
      "ref": [
        "const",
        "const"
      ],
      "rows_examined_per_scan": 7,
      "rows_produced_per_join": 7,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "1.55",
        "eval_cost": "0.70",
        "prefix_cost": "2.25",
        "data_read_per_join": "5K"
      },
      "used_columns": [
        "object_id",
        "field_id",
        "value"
      ]
    }
  }
}