Extreme slowing down of MacBook Pro due to pkd process

One or more of the plugins you have installed is misbehaving. The pkd pluginkit daemon can be queried with the following command to list all the plugins.

Open Terminal and run this command:

pluginkit -mAvvv > ~/Desktop/PluginKitReport.txt

Look on your Desktop for "PluginKitReport.txt" open the file in a text editor of your choice. The output listing will be extensive. Look for anything weird that isn't obviously a part of macOS provided by Apple. You can also run EtreCheck which will query your system and build a report detailing what system / kernel extensions you have installed as well as non-Apple background launchd daemons running. It does not detail personal info. Update your question with the output from EtreCheck.

Suspect a rogue adware browser extension or other adware / malware or perhaps some 3rd party software misbehaving.