SELECT 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  feature_id = 18 
  AND product_id = 16340 
  AND lang_code = 'en'

Query time 0.00041

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.001792605,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_features_values",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY",
            "fl",
            "lang_code",
            "product_id",
            "fpl",
            "idx_product_feature_variant_id"
          ],
          "key": "lang_code",
          "key_length": "12",
          "used_key_parts": ["lang_code", "feature_id", "product_id"],
          "ref": ["const", "const", "const"],
          "loops": 1,
          "rows": 1,
          "cost": 0.001792605,
          "filtered": 100,
          "attached_condition": "cscart_product_features_values.lang_code = 'en'",
          "using_index": true
        }
      }
    ]
  }
}