Hope you are doing well and sorry for the trouble. As described commenting PDF documents became laggy.
Is this a behavior with a particular PDF file or with all the PDFs? Please try with a different PDF file and check. If the file is stored on a shared network/drive, please download it to your computer first and then try again.
What is the version of the OS and the Adobe Acrobat DC you are using? To check the version go to Help > About Acrobat and make sure you have the latest version 21.07.20091 installed. Go to Help > Check for updates and reboot the computer once.