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With the patch UTF-8 decoder 20% slower for some data. UTF-16 decoder 20% faster for some data and 20% slower for other data. UTF-32 decoder slower for many data (even after some optimization, naive code was up to 50% slower). Standard charmap decoder 10% slower. Only UTF-7, unicode-escape and raw-unicode-escape have become much faster (unicode-escape and raw-unicode-escape as with issue16334 patch).
A well optimized decoders do not benefit from the _PyUnicodeWriter, only a slight slowdown. The patch requires some optimization (as for UTF-32 decoder) to reduce the negative effect. Non-optimized decoders will receive the great benefit.