Performance Matters
This sounds like a clear case of consultant software. <i>The people building it won't be dogfooding the result</i> - at most they'll be clicking around the UI on their powerful desktop machine and patting themselves on the back because it responds in less than half a second. <i>The requirements will be vague,</i> because getting performance requirements anywhere beyond stupidly vague "performant on modern hardware" or some shit requires <i>real expertise</i> in UI design and will cost <i>real money</i> to build, and UI designers won't be involved in the details until the contract is signed.<p>"Premature optimization" in reality is a vacuous phrase, because premature <i>anything</i> is bad - it's right there in the word! (Insert joke here.) The problem is that most developers (myself included in many cases) are just not qualified to say when it is premature to optimize, <i>because the requirements do not state anything meaningful about performance.</i><p>If you think this is overly pessimistic, I would encourage running pretty much any Android app or opening any major website on a 2018 or older smartphone. It's pretty obvious developers don't know how to build software for the probably 50% or more of customers who don't buy a new top-of-the-line phone every year.<p>As for performance guidelines, have a look at Jakob Nielsen's amazing <i>evidence-backed</i> UX guidelines such as Response Times: The 3 Important Limits[0]. The ones which should stick are that 0.1s feels instantaneous (probably not when using a scroll bar, but that's another matter) and 1s is the limit for not interrupting the user's flow of thought. In other words, if <i>anything</i> your program does takes more than 0.1s to respond to user input on the target hardware that should at least be acknowledged and prioritized (maybe at the end of the backlog, but at least then it's a known issue).<p>[0] <a href="https://www.nngroup.com/articles/response-times-3-important-limits/" rel="nofollow">https://www.nngroup.com/articles/response-times-3-important-...</a>