I remember refactoring some code to improve readability, then observing something that was previously a few microseconds take tens of seconds.
The original code created a large list of lists. Each child list had 4 fields each field was a different thing, some were ints and one was a string.
I created a new class with the names of each field and helper methods to process the data. The new code created a list of instances of my class. Downstream consumers of the list could look at the class to see what data they were getting. Modern Python developers would use a data class for this.
The new code was very slow. I’d love it if the author measured the time taken to instantiate a class.