>As a test we have upgraded a machine with SQL server 2008R2 to Windows 10 this morning.
>
>Prior to the upgrade SQL server performed fine. However after the upgrade especially table scan operations where one or two orders of magnitude slower.
>
>Any idea, what could be at play here? The specs of the machine are just fine intel I5, 3.1Ghz and 8 Gb of RAM. The task manager does not reveal anything stressing resources.

At first, this could be a cache issue. Whenever you restart the SQL Server service, or the same when you reboot the machine, SQL Server will not be fully optimized. This could take hours, depending on the amount of memory you have, and allocated to it. So, at first, it could be slow. Then, as time moves ahead, you might see improvements at this performance level.