If you post a question to the psql-performance mailing list, that suggests your system might be overloaded the first thing you'll be asked for is a snapshot of vmstat data. It's the most valuable quick summary of what your system is doing. Because it displays a full system snapshot per line, it's even possible to extract short-term trends from staring at a screen full of data.
Since the output from vmstat is a bit too wide to fit on the page at once, it's broken up into a left and right side for now; later examples will include just the interesting columns. Here's the left side showing a few seconds of heavy memory-limited pgbench work:
$ vmstat 1 procs -----------memory------------- ---swap-- r b swpd free buff cache si so 8 0 0 2542248 ...
