There will be impact for sure, but as long as you still perform your job to the satisfaction of both employers, then it does not matter.

If both employers know you are doing it and are ok with it there’s no problem. Its when it’s being hidden from them that it’s a problem. I’ll give you an example:

So say I’m a lead collaborating on a project with two people under me. Bob does great work, needs little to no assistance, and I can trust that he gets it done not only right, but also takes ownership of it and will bring things up if he sees problems. He doesn’t just wait for me to find them as the lead. Jim on the other hand takes a lot longer to do his part. He doesn’t bring up anything like Bob does even though it should be pretty obvious that it’s wrong because it impacts his part of the work, he waits for me to review, find it, and then have to reach back out to him for it to be fixed or changed. He is also far less responsive than Bob to emails, IMs, and isn’t able to multitask nearly as well. So clearly in this situation Bob is a better employee than Jim, and Bob receives bonuses and perhaps higher pay depending on other factors than Jim. However that’s not to say Jim is a bad employee and should be fired does it? A lot of the work can be subjective. Maybe Bob found those errors because he has more experience in the job and has run into the problem in the past and Jim has not. I can’t put “find problems in documents” in their job description so vaguely and then dock them every time they miss something, that would be absurd. People are different. Bob will likely be promoted to a higher role shortly, while Jim won’t, even though they were hired at the same time for the same role. I’d like to see Jim take more ownership of his work and take it upon himself to not just wait to be told what to do on a micro-level but come to me with what he thinks needs to be fixed, and so on. These are things that can be covered at reviews, and often have, and for a little while he seems to get better at it.

So given this situation, am I wrong in keeping Jim as an employee? His work isn’t as good as his peer who was hired for the same job at the same time, and he hasn’t stepped up to take on the same responsibility in the work that I would expect someone being on the job as long as he does. Is the right thing to do to fire him outright?

But now what If It comes to my attention that during the time he was working on the project he was working a second job, and not focusing on the task he told me he was working on. Did he not find those errors because he wasn’t paying attention due to working on something else? Was he avoiding any added work from found errors so he could do work for that other company? Is his lack of multitasking ability not actually true and he’s doing work for someone else while ignoring me further delaying the project and taking my time up to help him?

“Well Enough for your standards” is entirely subjective. I don’t treat my people like robots who can be mathematically judged. There isn’t a binary existence of good or bad. I don’t fire people immediately upon feeling they are sub-standard. I feel I often bend over backwards trying to help people get better, I push for bonuses and promotion when I think they are deserved. I don’t at all believe that outcome that is all that matters. I truly believe that a person who puts in more effort and gets a worse result will be a better employee long term. I’ve seen it time and time again.

And to your last point about paying people enough; While that’s certainly true in some cases, the article cites tech workers pulling in $500,000 through it. Clearly it’s not about being under paid.