A Quickbooks user must make sure that the minimum system requirements are completed as listed below
Minimum  system requirements to Avoid QuickBooks Running Slow Problem

QB Desktop Pro comes with a minimum of 1 user and a maximum of 3 users license, similarly, Premier 1 to 5 users and Enterprise offer 1 to 30 users. (Runs on Windows 10, 8.1 and Windows servers 2019)

Requires a minimum of 4 GB ram for a single user or more depending on the number of users accessing the program at the same time, For eg., you will need 8 GB of RAM for 1 to 3 users to access company data folder at the same time and so forth the ram needs to be increased as the number of users increase in your organization.

https://accountingerrors.com/quickbooks-running-slow-fix/

https://www.townscript.com/e/why-my-quickbooks-running-slow-220144

http://ttlink.com/qbrunningslow

https://www.scoop.it/topic/qbrunningslow