There are factors to consider so that QuickBooks Online can perform smoothly. One of which is the system requirements. To learn more about it, please see this article: System Requirements for QuickBooks Online.
Another cause of slow performance is due to some browser-related issues. There are times that cache can't deliver faster access of data. To help speed up browsing experience, we recommend deleting cache's temporarily historical data. You can check the details on how to do it in this article: How Do I Clear My Browser Cache and Temporary Internet Files?
If you need anything else, please let us know. We'll be right here to help.