SELECT 
  * 
FROM 
  cscart_product_feature_variants 
WHERE 
  variant_id IN (5908, 1241, 5916)

Query time 0.00057

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": 3,
      "filtered": 100,
      "index_condition": "cscart_product_feature_variants.variant_id in (5908,1241,5916)"
    }
  }
}