SELECT 
  feature_id 
FROM 
  cscart_product_features 
WHERE 
  feature_id IN (343, 344, 346, 348) 
  AND feature_type = 'E'

Query time 0.00018

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.007671046,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_features",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["feature_id"],
          "loops": 1,
          "rows": 4,
          "cost": 0.007671046,
          "filtered": 100,
          "index_condition": "cscart_product_features.feature_id in (343,344,346,348)",
          "attached_condition": "cscart_product_features.feature_type = 'E'"
        }
      }
    ]
  }
}