We thought we fixed this issue, but we're still getting reports that users are seeing AdBlock at 100% CPU in Activity Monitor. We're continuing to investigate.
Disable the AdBlock Icon extension in Safari > Preferences > Extensions. That will return your CPU usage to normal.
With the AdBlock Icon extension disabled, you can still do pretty much everything you might need to do using either the AdBlock icon in the MacOS menu bar or in the AdBlock app itself. Please see these articles in our Knowledge Base: