Hiring a Senior Developer to make things faster is a wrong role assignment.
While it can be possible when hiring one for a greenfield project, you can utilize the experience and know-hows on early stages, bringing a value by doing things "right" from the beginning, many companies think if they hire someone with 10 years experience they can gain x 10 velocity, which is an illusion.
First, context matters: Developer who was working for some company has his experience scoped to that company. The context, processes, the way of work, constrains and etc will take time to get used to.
Second, it takes time to freely swim in someone else code event if code is a "piece of art".
This doubtful assumption usually leads to missing expectations and contra-productivity.
It makes sense hiring a Senior Developer to trigger changes, but requires consciousness, mood and will.
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