The Surface tablet includes Windows Defender as a service which causes a slight performance hit. Not major but it’s something that doesn’t need to be running (in my opinion).
To turn it off:
1. Open regedit. Find HKLM\SOFTWARE\Microsoft\Windows Defender
2. Take ownership of the key and add write permissions to yourself (Right click Windows Defender key, click Permissions, click Additional settings, click Owner, type your username and accept. Now on the Permissions window click Add, type your username and check Full Access and click OK)
3. Change the value of DisableAntispyware to 1