I have installed ArcGIS 10.5.1 on four different Windows 2016 servers. The 64Bit Python and 64 bit Background processing have also been installed on these machines. After noting what appeared to be python slowness, I have narrowed down the issue to Python that is connecting to SDE (SQL Server). A simple job that deletes 65k rows and then appends 65k rows from one SDE layer to another can take up-to 12min to complete.
The same job on my Windows 7 machine with the 64 bit libraries and ArcGIS 10.5.1 takes about 2.5 min, every time I run it. The same job on a windows 2012 R2 server with the 64bit libraries and ArcGIS 10.5.1 takes about 2.5 min every time. The Windows 2016 Servers run the code sometimes in 2.5 min but most of the time it takes greater than 8 min up-to 12 min.
I have created a support request with ESR, I hope they come back with something soon.
Meanwhile, has anyone noticed an issue with Windows 2016, Python and insert/delete of SDE layers?