SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (15753, 15754, 15755, 15756) 
  AND feature_id IN (549) 
  AND lang_code = 'en'

Query time 0.00028

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.003259455,
    "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": "lang_code",
          "key_length": "12",
          "used_key_parts": ["lang_code", "feature_id", "product_id"],
          "loops": 1,
          "rows": 4,
          "cost": 0.003259455,
          "filtered": 100,
          "attached_condition": "cscart_product_features_values.feature_id = 549 and cscart_product_features_values.product_id in (15753,15754,15755,15756) and cscart_product_features_values.lang_code = 'en'",
          "using_index": true
        }
      }
    ]
  }
}