Based on my research, there's no new documents for Exchange 2019 performance. However, it is mentioned in the official document that "Exchange 2016 and Exchange 2019 continue the features that were introduced in Exchange 2013 to help you manage server health and performance". So you can check documents provided in Exchange 2013 for Exchange 2019 server health and performance.
Server health and performance in Exchange Server
And you could refer to the link which introduces about: Working with Performance Counters in PowerShell
Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.
If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.