This is nonsense. You're micro-benchmarking some detail without reflecting on how to use these methods. I take it you have this code (sort of) const all = [[

This is nonsense. You're micro-benchmarking some detail without reflecting on how to use these methods. I take it you have this code (sort of) const all = [[1,2,3,4,5], [1,2,3,4,5], ...] //15000 items let ret = []; for (item of all) { ret = ret.concat(item); // Or ret.push(...item); } The point of using concat is of course that you don't have to do the loop. You're shuffling bytes for no use like crazy there. Not concat's fault. You should be doing: [].concat(...all); This runs in ~1ms on my machine (latest Chrome). Custom versions, and loop + push both come it at around ~4ms.