We are sorry for the trouble. As described, the application is slow when you open the PDF in protected mode.
Would you mind sharing the version of the Adobe Acrobat DC you are using? To check the version go to Help > About Adobe Acrobat and make sure you have the latest version 20.12.20043 installed. Go Help > Check for Updates.
Is the file is stored on a shared network/drive, please try to download the file to your computer locally and then try to open it with Acrobat DC and check.
For testing purposes: Please try to turn off the protected mode. Go to Edit > Preferences > Enhanced Security >Uncheck 'Enable protected mode at startup', Turn off the protected mode and uncheck the enhanced security > Click OK and reboot the computer.
Note: Turning off the security may possess security risks, please turn on the security after testing.
If turning of the security works for you, please try to add the file and the folder path to the Acrobat whitelist. Go to Edit > Preferences > Enhanced Security > Add file and Add folder path > Click OK and reboot the application.