For me, its the time investment needed to understand enough of something new for it to actually be useful. Trying to find time around work and life commitments, its hard to find the few hours I need to get the next step down the path of learning something new.
For example, trying to contribute to DEV on GitHub, learning a whole new ecosystem has now eaten 3 full days of my time for very little progress. Fortunately the nature of my work means that I have that time free at the moment, but its unlikely I will have this amount of time again for a long time.
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