What is Safari Service worker ? Would disabling it affect website performance?
Service Workers can be used by web sites to handle functionality such as:


Enabling Offline mode when you have no Internet connection
Receiving Push Notifications
Caching data files for faster load times
Synchronizing data in the background to provide streaming updates
Trigger data refreshes based on for example location changes


The impact of disabling the feature depends on the sites involved. Some would just continue working as normal, but perhaps work a tad slower - others would loose functionality, such as offline mode or push notifications - and again others would stop working completely as they insist on that feature working.