Recovery from this state of affairs doesn't happen by telling people how to fix concrete, individual things, but teaching them to think about what they are doing and to analyze things. In order to know when you are off by orders of magnitude you need to have some idea of what is reasonable before you measure. If the reality you achieve is way off what is reasonable, you either need to change your perception of what "reasonable" is or you have to face the possibility that you may not have done a good job.
A chat client that deals with perhaps 3-4 teams and perhaps 200-300 people should not require 1Gb of memory and it should consume a negligible amount of CPU. Any way you slice it that's a LOT of resources per user, per team or per communication volume. So Slack miserably fails even cursory glance at workload/resource usage.
You can start off with the lower layers: speaking a chat protocol. How much resources should that eat up? For a chat application: too little to measure on a modern machine. (I spent most of the early 2000s working on web-scale web crawlers on much, much weaker server machines than even cheap desktop machine is today). The IO should have trivial cost, the protocol parsing should likewise have trivial cost or you have messed up badly.
Then move on to the internal state. Draw it on paper. What are the main structures you will be needing. How much data do you need to keep in memory, how much space does it need to take, how do access it and manipulate state? What is a reasonable CPU expenditure?
Next is the UI. How do you make it responsive. How do you make it not gobble up tons of memory. Are there tradeoffs you can make? How do other applications do things? Where are the resources wasted?
One doesn't produce a resource hog like Slack by doing one or two things in a wasteful and sloppy manner. One does it by having the wrong attitude and systematically doing a bad job.