Pretty decent thing to do. When I was a website developer, around 2012 I dealt with a mentally unstable friend/client who one day started seeing porn ads on a blog that he asked me to fix up on behalf of someone else. He freaked out “NICK CAN’T SEE THAT, OMG”. I looked at the site on 4 different devices from 3 IP addresses and didn’t see what he was talking about. Since it was my job to figure this out, I asked some people we knew on a forum “will you please look at this site and let me know if you see any porn ads?” 25 people responded that no, they did not. Okay. So I told him, hey, I had everyone on our forum check it out and it’s fine, so it has to be a virus on your computer or something. He got super angry and told me it was VERY embarrassing to him that I did that (like, wtf was I supposed to do?). Eventually someone figured out he had a hacked router.
As far as transactions, yes, something like Visa takes way less than Bitcoin. For HFT compared to crypto, who knows, but one thing they’re not doing is something intentionally wasteful like proof of work.