I mean, I can only refer to what I learn(ed) in my own pilot training where it was explained explicitly as not a general order of operations, but an order of priorities to keep in mind, as I laid out above. I did a quick Google search on the phrase to see if I could find a more definitive answer on how it is meant and even though I didn’t, most articles seem to focus on the “don’t put your sole focus on troubleshooting some small issue but always make sure you are still actively flying the airplane, avoiding terrain/other traffic, talking to ATC/other aircraft where applicable, etc.”
From an ATC perspective, I can only tell you how annoying it is to have to wait for readbacks. Particularly in busy areas (and the problem only gets worse the “higher” you go because VATSIM’s topdown and training model has the unfortunate side effect that there are many aerodrome controllers, some approach controllers, and only few center controllers, whereas in the real world the distribution is usually the exact opposite way around), you regularly have a multitude of instructions lined up and having to wait ages for a readback means you have to wait giving these other instructions as well (I mean, you could give them regardless but that will cause a bunch of confusion on when pilots have to give their readback). That’s why many, if not most, controllers will usually call you up again if you didn’t respond for five or six seconds because at that point the assumption will be that you probably didn’t hear the instruction. This occasionally leads to the readback then being blocked out by the reiteration of that instruction, but much more often than not the pilot only gives the readback after being called up a second or third time. Were they actually not paying attention or just taking a lot of time to set their MCP before giving the readback? Who is to know; from an ATC perspective it really doesn’t matter. In this regard I also don’t really agree with this statement:
I certainly can’t speak for all VATSIM controller clients, let alone all real world controller software, but at least in EuroScope, the scope’s update rate is locked at five seconds (and while some ground, tower, and approach facilities are now getting advanced radar systems that can update roughly every second, an update rate of about five seconds is - to my knowledge - quite realistic for most facilities and you of course need to keep in mind that not all facilities actually have surveillance capabilities so in those cases ATC has absolutely zero way to see whether a pilot is complying with an instruction). This means that it can easily take 10-15 seconds before the scope starts indicating your turn/level change/ speed change/etc. and that doesn’t even tell me whether you correctly understood the instruction.
Sure, there are situations where you should start complying with an instruction before reading it back if you can’t do both at the same time, but unless it’s absolutely obvious to the pilot (or at least should be - this is also where my jab at pilot competency was aimed at because somehow people always end up surprising you with how unaware they are of even their own situation…), such as when you are on a tight base and get your intercept heading, any somewhat competent controller will append or prepend the respective instruction with a term like “immediately” or some further context like “due to traffic”. In all these situations, controllers will also know that it might take you longer to give a readback.
And if you not immediately following a normal instruction causes a conflict, then, frankly, the controller messed up big time (and even if they use “immediately” or similar in the instruction, these situations only occur if either ATC messed up or some pilot is not following their instructions, e.g. descending through the cleared level if you are just 1000ft below that cleared level). Luckily VATSIM is a virtual environment and we are flying/controlling pixels, not actual people, so if you actually end up hitting terrain or another aircraft, nobody will end up hurt (except maybe the pride of whoever messed up) - if you feel that it had a significant impact on your experience (which is of course a very subjective thing), then the best thing to do is probably to send feedback to the vACC so they can talk about the situation with the controller and give them additional training if necessary, because this is definitely not a situation that should ever occur (and if you are situationally aware enough to know that it is an immensely time-critical instruction, then I think it’s perfectly acceptable to comply first and read back later - same logic as before: ATC will know that this is the case).
And in regard to complying with instructions while reading them back, I wrote down some advice in this thread some time ago that should work regardless of one’s setup:
Something you can do here would be to have the mouse already on the heading knob in anticipation of the turn, start turning in the instructed direction while ATC is giving the instruction, then once ATC has finished talking, immediately read back, and only after that finish turning the heading knob (unless you can get it set up completely while ATC is still giving the instruction, of course) - that way you already start the turn and continue turning while reading back.