SQL Server TempDB files 1-> n and write stalls growing
The information from sys.dm_io_virtual_file_stats is aggreagated since the database came online.  

To get usable IO numbers from sys.dm_io_virtual_file_stats, you really need to capture a baseline, wait for a period of time (say 30 minutes) and take another snapshot of sys.dm_io_virtual_file_stats. The difference between the snapshots is the relative latency during that time.

This script take a baseline snapshot of sys.dm_io_virtual_file_stats, waits 30 minutes (by default) and takes another snapshot of sys.dm_io_virtual_file_stats.  The difference between the snapshots is the relative latency during that time.