SELECT 
  feature_id 
FROM 
  cscart_product_features_values 
WHERE 
  variant_id = 2343

Query time 0.00015

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.067419468,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_features_values",
          "access_type": "ref",
          "possible_keys": ["variant_id"],
          "key": "variant_id",
          "key_length": "3",
          "used_key_parts": ["variant_id"],
          "ref": ["const"],
          "loops": 1,
          "rows": 54,
          "cost": 0.067419468,
          "filtered": 100
        }
      }
    ]
  }
}