In 60 seconds you can get a high level idea of system resource usage and running processes by running the following ten commands. Look for errors and saturation metrics, as they are both easy to interpret, and then resource utilization. Saturation is where a resource has more load than it can handle, and can be exposed either as the length of a request queue, or time spent waiting.
dmesg | tail
mpstat -P ALL 1
iostat -xz 1
sar -n DEV 1
sar -n TCP,ETCP 1
Some of these
require the sysstat package installed. The metrics these commands expose will help you complete some of the USE Method: a methodology for locating performance bottlenecks. This involves checking utilization, saturation, and error metrics for all resources (CPUs, memory, disks, e.t.c.). Also pay attention to when you have checked and exonerated a resource, as by process of elimination this narrows the targets to study, and directs any follow on investigation.
The Netflix Tech Blog: Linux Performance Analysis in 60,000 Milliseconds http://techblog.netflix.com/2015/11/linux-performance-analysis-in-60s.html
Lots of good info here, though I suspect many sys admins already run through most of this once they land on a box.