I can share some insights into why you're experiencing some loading pages with invoices and estimates in QuickBooks Online (QBO), Keep.
Since you're experiencing speed issues with QBO, there are factors to consider to enhance user experience:
The computer or device's processing power
The amount of available memory (RAM)
The bandwidth of your internet connection to perform as intended
A lack of any of the mentioned areas can lower performance. In the meantime, let's verify if your system meets the basic requirements to narrow down the cause of slow performance: System requirements for QBO.
Additionally, please refer to this article for the step-by-step process to improve performance when using QBO: Performance Overview for QBO.