> because just iterating over all the pixels in an image by using "for(let i=0;i< pixel.length;i++){}" is 3x slower than using var, causing just a single full loop to be 80-120ms on a good computer.

That is an implementation detail. Of specific chrome versions.

That does not make let itself bad practice. In principle it should be significantly easier to optimize for a compiler because its more limited scope. So in the long run or possibly on other browsers it is likely best practice.

Putting transients before long-term behavior leads to urban programming myths and legacy baggage.

In fact in some jsperf benchmarks on chromium let happens to be faster for me.