Perceived performance is the cornerstone of multi-tasking on computers, i.e. doing more things at once than you have processors available.
two general points though:
- further evidence that over-optimising one aspect leads to a sub-optimal whole.
- users are fickle.
To me it doesn’t matter if the new copy is 2% faster, I just want to know what is actually happening. 10% would start to change things, but at that point, the problem is probably not progress reporting.