In this case, it wasn't too bad because:
In this case, most of the loss of velocity came from things like the adjustment to React and explaining the downright awful application structure. It was definitely worth bringing in someone else - it'd take me a lot more time and hassle to do a much crappier job of the front end.
However, if it had been a case of bringing in another back-end developer to work on similar tasks to me, I'd have had to spend a lot of time explaining what I could, going through the awful parts of the code base, and avoiding stepping on each other's toes.
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