I finished watching a video about pipelining https://www.youtube.com/watch?v=eVRdfl4zxfI which I thought made sense. Latency is the amount of time it takes to complete each instruction. Even with pipelining, the time it takes to complete an instruction is still the sum of the time it takes for every stage, in this case it's 20ns.
But then I did a bit more studying and found out the latency is supposed to be time for the longest stage to finish execution + the "cost" of pipelining, rather than the sum of time it takes to execute all the stages? http://web.cs.iastate.edu/~prabhu/Tutorial/PIPELINE/perform.html
Is there something I'm missing?