Edit: Mitch Faatz has pointed out in the comments that this may not be the best example. When testing, he determined that using the spread operator is three times slower than using
concat. For larger data sets using the spread operator in this way would not be ideal.
Experts often don’t know the problem any better than you. What they learned is not to trust themselves. They dig deeper and keep reducing the test case even if nothing obvious jumps at them for several hours. They know there is a problem somewhere, and finding it is a boring, mind-numbing but usually finite process.