With sp_BlitzFirst & sp_WhoIsActive in a SQL Agent job with scheduled execution for every 10-15 minutes, you can look back in time in terms of What was running, its execution stats, file stats, wait stats and Perfmon counters. This would help you to answer anybody as why your server was slow at a particular point in time.
Click through for the post and check out the GitHub repo as well. Baselining is extremely important for proper administration.