SELECT 
  profile_fields_data.field_id, 
  profile_fields_data.value, 
  profile_fields.field_type 
FROM 
  cscart_profile_fields_data AS profile_fields_data 
  LEFT JOIN cscart_profile_fields AS profile_fields ON profile_fields_data.field_id = profile_fields.field_id 
WHERE 
  object_id = 101 
  AND object_type = 'U'

Query time 0.00066

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "profile_fields_data",
      "access_type": "ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "6",
      "used_key_parts": ["object_id", "object_type"],
      "ref": ["const", "const"],
      "rows": 11,
      "filtered": 100,
      "index_condition": "profile_fields_data.object_type = 'U'"
    },
    "table": {
      "table_name": "profile_fields",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["field_id"],
      "ref": ["dev.profile_fields_data.field_id"],
      "rows": 1,
      "filtered": 100
    }
  }
}