In your highlighting example, instead of working with someone who will optimize, I'd much rather work with the guy who will realize we can just debounce the dialog, write a one line comment about the performance issue, and then move on to the next thing.
Maybe I'm biased though. I've worked with the performance guy; he was brilliant and the code was clever and highly performant, but he was also slow and didn't write clear, maintainable code. We didn't ship in time and I had to find a new job when the project got scrapped. Damned if it wasn't fast though.
As long as you are aware of the issue with naive string concatenation, you can use a string buffer or a mutable string – it is trivial to solve in a few more lines of code. What's more difficult involves deletion in the previous text. If this happens often, you will need rope or skip list, which is challenging if you implement from scratch. In the latter case, I would leave a comment without implementing the optimal solution and let the profiler decide later.
Sounds more like a project management issue to me, than a problem with that coder.
I'd rather see basic project management skills than basic knowledge of algorithms. Unfortunately, the former is much more rare.
My conjecture is that a big fraction of programs could get significant performance boost noticeable by end users if they were developed by good developers who have some knowledges in algorithms. You don't need to be an expert, but knowing very basics on time complexity and fundamental data structures is necessary to every programmer IMHO.