Troubleshooting performance issues is already quite large task with many different areas to check.
avoiding performance issues even more so.
Is there a specific slowness in their environment, and if so where?
After collecting more information and checks the outcome and details of these checks can be viewed. Also most important when talking about system performance is to have clear numbers, based on exact tests performed at the same time and in the same manner.
only if this procedure is followed and evidence collected will you be able to know if the system performs normal or not.
Please also have a look at the below report which might help.