How many software projects have you seen fail because it couldn't run fast enough or used too many resources? Personally, I've never seen it. I'm sure it exists, but I can't imagine it's a common occurrence. I've rewritten systems because they grew and needed perf upgrades to continue working, but this was always something the business knew, planned for and accepted as a strategy for success. The project may have been less successful if it had been written with performance in mind from the beginning.

With that in mind, I can't think of many things less appropriate to keep in your mind as a first class concern when building software than performance and optimization. Sure, as you gain experience in your software stack you'll naturally be able to optimize, but since it will possibly never be the reason your projects fail and presumably your job is to ensure success of some project, then it follows that you should prioritize other things strongly over optimization.