Slow SQL Server authentication, but fast SAN, fast querying?
Here is your answer:

MSDB on C:
86% of 32GB
io_stall_write_ms : 192217


If MSDB is on c: master is on c: as well and I'm almost willing to bet that tempdb is still in the default location as the average I/O stall on c is 19 seconds!

First, check if tempdb is on c:\ or has any file on the c:\ partition and if so move it away

Check if the master database has some free space - Usually you dont need to tune it as very little is written to the master database, unless if someone has created objects in it

Then make space in the MSDB database is way to big which can cause a lot of troubles, please remove as much backup and job history from it database as you can. 

You can do all these checks very simply by downloading and running sp_blitz