I ran decodebench.py and bench-diff.py scripts from #14624, I just
replaced repeat=10 with repeat=100 to get more reliable numbers. I
only see some performance regressions between -5% and -1%, but there
are some speedup on UTF-8 and UTF-32 (between +11% and +14%). On a
microbenchmark, numbers in the -10..10% range just means "no change".

Using _PyUnicodeWriter should not change anything to performances on
valid data, only performances of handling decoding errors between the
overallocation factor is different, the code to widen the buffer and
the code to write replacement characters.