No, our Marketplace module does not affect any kind of performance issues with the standard Opencart. All of our extensions are built on standard Opencart MVC(Model View Controller) ORM(Object Relational Mapping) design pattern. We also follow the best practice of extension development so that our extensions are as scalable as is your standard Opencart.
Please do contact us for any further queries or requirements. You can also check our Opencart Extensions for additional features.