A benchmark tells you how long the whole thing took. It doesn't say why. For that you need to see where the time goes.
Profiler
A tool that watches a program run and reports how much time each function took. Self time is what the function spent on its own. Total time also includes everything it called. The answer often surprises people.
This is the most useful thing you can hand an agent for speed work. Without a profile it optimises what looks suspicious. With one it starts from a list of where the time went, and you can check whether its change moved the top line.
Say it to your agent
“Run node --cpu-prof scripts/report.js, read the .cpuprofile, and list the top five functions by self time with their percentages. Don't edit anything until I have seen that list.”