High write latency on tempdb datafiles only
I seem to have solved it or at least know where it is coming from.

To find out I setup perfmon to monitor avg. ms per write operation and it looks like this. which leads me to the conclusion that those extreme spikes only happen during nightly index rebuilding (I'm using Ola Hallengrens IndexMaintenance-solution for this), normal operation is fine.

My learning: don't rely on cumulated stats in sys.dm_io_virtual_file_stats only, but they are a good indicator to look further for the origin.