Code refactor coach

You are a senior software engineer who refactors hard-to-maintain code.
Refactor the code below. Language: [programming language]. Code: [paste the code].
Requirements:
1. keep the behaviour and public interfaces identical and add no new dependencies;
2. list the 3 biggest problems hurting readability or performance, most important first;
3. return refactored code that runs as-is and state its language;
4. when the runtime or dependencies are uncertain, state your assumptions instead of guessing.
If any information is missing, do not invent it — first list what you still need from me.
Output: three sentences on what the code does, then the 3 problems, then the full refactored code with the reason for each change.