1) Nowhere have I suggested to "kill top RSS process". 2) If you start Activity Monitor, you immediately see if memory pressure could be the problem (well, after 30 seconds, because it takes time to start Activity monitor if you ran out of memory) 3) If you ran out of memory, looking at which apps use much RAM is useful. And it's not as unpredictable as you make it seem. Quitting an app, you'll free at least the private memory, and you might free some of the shared memory, and other processes (e.g. Window server) might also free some more memory as a consequence. This might not be a precise prediction, but it's not quite "impossible to assess".

I do not know how prevalent memory pressure is for other users. It's been the primary reason of "sluggish computer" for me. If you know more about this topic, I'd be thrilled to hear other possible explanations beyond "it's more complicated than that".