SELECT 
  * 
FROM 
  cscart_product_feature_variants 
WHERE 
  variant_id IN (
    1449, 1201, 1219, 5899, 1257, 5902, 5908, 
    1313, 1280, 5909, 1200, 1198, 1199, 
    1241, 5916
  )

Query time 0.00038

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_feature_variants",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["variant_id"],
      "rows": 15,
      "filtered": 100,
      "index_condition": "cscart_product_feature_variants.variant_id in (1449,1201,1219,5899,1257,5902,5908,1313,1280,5909,1200,1198,1199,1241,5916)"
    }
  }
}