SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    18508, 18509, 18510, 18511, 18513, 18514, 
    18515, 18516, 18517, 18518, 18519
  ) 
  AND feature_id IN (549, 596) 
  AND lang_code = 'en'

Query time 0.00037

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.01843278,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_features_values",
          "access_type": "range",
          "possible_keys": [
            "PRIMARY",
            "fl",
            "lang_code",
            "product_id",
            "fpl",
            "idx_product_feature_variant_id"
          ],
          "key": "fl",
          "key_length": "9",
          "used_key_parts": ["feature_id", "lang_code"],
          "loops": 1,
          "rows": 92,
          "cost": 0.01843278,
          "filtered": 23.91304398,
          "attached_condition": "cscart_product_features_values.product_id in (18508,18509,18510,18511,18513,18514,18515,18516,18517,18518,18519) and cscart_product_features_values.feature_id in (549,596) and cscart_product_features_values.lang_code = 'en'",
          "using_index": true
        }
      }
    ]
  }
}