Wouldn't this layer of dust also stop the lense from working normally? If I understand the other commenters here, it's not just about reversibility, but "useful energy" loss - with lenses you are not paying the needed "efficiency tax" to achieve higher temperatures than the source.
Interesting. I did add a return to that benchmark and it didn't seem to affect the results. I would think your test would be identical to mine in performance characteristics, I wonder if it's just the benchmarking methods leading to the difference. I might just go write my own benchmarking like yours since I don't really know what measurethat.net is doing.
It seems intuitively obvious that map would be less efficient, but who knows what V8 is doing to optimize under the hood.