SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    376, 1612, 1613, 1614, 1615, 1616, 1617, 
    1618, 5476
  ) 
  AND feature_id IN (78) 
  AND lang_code = 'en'

Query time 0.00053

JSON explain

{
  "query_block": {
    "select_id": 1,
    "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": "PRIMARY",
      "key_length": "6",
      "used_key_parts": ["feature_id", "product_id"],
      "rows": 9,
      "filtered": 77.778,
      "attached_condition": "cscart_product_features_values.feature_id = 78 and cscart_product_features_values.product_id in (376,1612,1613,1614,1615,1616,1617,1618,5476) and cscart_product_features_values.lang_code = 'en'",
      "using_index": true
    }
  }
}