Microsoft recently rolled out a new feature “Startup Boost” to Microsoft Edge stable channel, which is the public release version of the browser. But, now Microsoft has disabled the feature in Edge by default. But the Startup Boost feature still remains there, it can be enabled manually if you want to use it.
What has prompted Microsoft to disable the feature is feedback by users. Microsoft has acknowledged that users have been facing some issues related to Startup Boost. It seems that some users are seeing the browser reopen immediately after exiting. Others see message about reopening tabs when startup boost is enabled.
It also has confirmed that it is working to resolve these issues. Microsoft has provided this information in Startup Boost FAQ.
March 19, 2021: Why is startup boost disabled in Settings when it was previously enabled?
Our team is working on fixing a couple of issues where some customers are seeing the browser reopen immediately after exiting and/or a message about reopening tabs when startup boost is enabled. Thank you to our Insiders who helped report these issues! If you would like to use this feature still, you can manually enable startup boost in edge://settings/system. We will update this FAQ when we believe this to be resolved.
Coming to how the feature works, Startup boost starts core Microsoft Edge processes in the background to keep them ready when the browser is closed, so Microsoft Edge will start more quickly when launched. It however keeps the browser running in the background with minimal processes. This way it has a limited impact on device resource usage. Startup boost does not add any additional resources when browser windows are already open.
Microsoft will update the FAQ when the issues are resolved. For the time being you can enable or keep the Startup Boost disabled if you face any issue while keeping it on.