Using a domain account that is in the local Administrators group in Windows 2008 or above DOES NOT mean you get the same privilege set that you get when running under NT Authority/System.
Windows 2008 and above have User Account Control, and one aspect of this is that accounts in the local Administrators group get two sets of privileges. By default they use the local User set of privileges. They only get to use the full Administrator set when the application is 'Run as Administrator'.
There is in fact no need whatsoever for the account running the SQL Server services to be a member of the local Administrators group. What is needed is for the account to be explicitly given the privileges that it needs to run SQL Server efficiently.
The Microsoft documentation has a ilst of the rights needed in various circumstances, but my consolidated ist would grant the SQL service account the following rights. Some of these are set up by the SQL install or SQL Configuration Manager, but others you need to grant explicitly. When you have done this, the domain account should perform at least as well as the Local System account.
Original author: SQL Server FineBuild 1-click install and best practice configuration of SQL Server 2017 2016, 2014, 2012, 2008 R2, 2008 and 2005. 1 Dec 2016
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