QuickBooks might lag or perform poorly if the system doesn’t meet the minimum system requirements, the internet connection is poor, or the file is too long. To optimize the performance of QuickBooks when working in multi-user mode, check your network connection and ensure it is strong and stable. Moreover, the server computer should meet the system requirements to run QB, as that will impact the performance. The size of the company file also has a huge role to play. If it grows to an incredible length, you may find QuickBooks running slow. Condense your company file to reduce the size and increase speed.

https://proadvisorsolutions.com/blog/quickbooks-desktop-running-slow/