Identify performance bottleneck in SQL Server, checked the usual suspects
I'm going to be brutally honest here: those are not the right metrics to look at, and wherever you're looking that gives you those metrics is wildly outdated. Those are not the usual suspects.

In the year 2018 (and indeed, for the last several years), the way you diagnose a slow SQL Server is by using wait stats. SQL Server is constantly tracking what queries are waiting on, and you can see it in the DMV sys.dm_os_wait_stats.

My favorite way to visualize that is with sp_BlitzFirst @SinceStartup = 1. (Disclaimer: it's my favorite because I wrote it, and it's free &amp; open source.)

Try editing your answer to include a screenshot of your wait stats from sp_BlitzFirst, and we may be able to get you a lot closer.