Honestly I'm kinda surprised there aren't handheld laser engraving devices made for quick and permanent graffiti.
I've experimented with using a 60 W (I think*) laser on a small steel bracket, and even with the beam holding on a single point for a minute, it made a barely visible dot that you couldn't feel by running your finger over.
* It was nearly a decade ago, but I looked up the relevant hacker space and unless they changed the model, it was 60 watts.
The bracket was around 1cm by 5cm, and around 1mm thick.
I know that a friend had a 3-5W laser (don’t remember -it was a CO2 laser) he used for wood burning, and it was fast.
It may also be what they measure.
The fiber marking LASERs at work are 1064nm, and at a mere 20w output, will absolutely eat away at steel with no problems.
Edit: I should note there are CO2 metal cutting LASERs, but they are at very, very high output powers to overcome that reflectivity barrier. You need 500w 10600nm to cut through what a 30w 1064nm could cut. My 80w CO2 barely cuts through heavy-duty aluminum foil, and in many spots it isn't a full cut. A 20w marking LASER at 1064 would obliterate the foil.
Everlast now has a laser welder, too.
I think it’s only a matter of time before we get cheaper cutter in the 1kw range.