One doesn’t even need to go as far as C++ or Rust… many best in class Mac apps were written in Objective-C (which is notably slower than either) and were plenty responsive on single core PowerPC G3s, G4s, and G5s.
I think a great deal of the issue comes down to bloated frameworks, bad application code, and a prevalent attitude of nonchalance towards optimization that compounds with dev machines being powerful enough to mask most performance problems.
I think it's probably more productive to think of this in economic terms; "nonchalance toward optimization" <-> "I can write this code in an hour instead of three hours". And, from a company's perspective: "I can get this code running acceptably at 1/3 the dev cost".
Surely we’ve crossed a threshold of diminishing returns on this by now though? There has to be a point where the gains in convenience/DX/speed are too tiny for the costs involved and the weight of the average app ceases to grow.
I think a great deal of the issue comes down to bloated frameworks, bad application code, and a prevalent attitude of nonchalance towards optimization that compounds with dev machines being powerful enough to mask most performance problems.