Benchmark: bench_case.py. Hum, case_writer.patch looks to be always slower:

--------------------+--------------+----------------
Summary             |         orig |          writer
--------------------+--------------+----------------
lower with 'a'      |  5.76 ms (*) |         5.76 ms
lower with 'é'      |  62.9 ms (*) |  76.8 ms (+22%)
lower with '€'      |  75.2 ms (*) |  83.6 ms (+11%)
lower with 'fi'      |  75.3 ms (*) |  83.7 ms (+11%)
lower with 'ß'      |  66.4 ms (*) |    76 ms (+15%)
upper with 'a'      |  5.66 ms (*) |         5.66 ms
upper with 'é'      |  48.3 ms (*) |  75.9 ms (+57%)
upper with '€'      |  50.1 ms (*) |  77.9 ms (+55%)
upper with 'fi'      |  93.7 ms (*) |   137 ms (+46%)
upper with 'ß'      |  91.9 ms (*) |   119 ms (+29%)
casefold with 'a'   |  5.66 ms (*) |         5.67 ms
casefold with 'é'   |  64.5 ms (*) |  95.8 ms (+48%)
casefold with '€'   |    67 ms (*) |  96.1 ms (+43%)
casefold with 'fi'   |  97.1 ms (*) |   132 ms (+35%)
casefold with 'ß'   |  93.7 ms (*) |   122 ms (+30%)
swapcase with 'a'   |  99.7 ms (*) |    107 ms (+7%)
swapcase with 'é'   |  99.7 ms (*) |    107 ms (+7%)
swapcase with '€'   |    78 ms (*) |  87.4 ms (+12%)
swapcase with 'fi'   |   143 ms (*) |    152 ms (+7%)
swapcase with 'ß'   |   140 ms (*) |          138 ms
title with 'a'      |    82 ms (*) |  98.2 ms (+20%)
title with 'é'      |  81.9 ms (*) |  98.2 ms (+20%)
title with '€'      |  90.2 ms (*) |   115 ms (+28%)
title with 'fi'      |  93.9 ms (*) |   112 ms (+20%)
title with 'ß'      |  91.3 ms (*) |   103 ms (+13%)
capitalize with 'a' |  62.3 ms (*) |  79.2 ms (+27%)
capitalize with 'é' |  62.1 ms (*) |  79.1 ms (+27%)
capitalize with '€' |  72.9 ms (*) |         76.5 ms
capitalize with 'fi' |  72.6 ms (*) |  90.3 ms (+24%)
capitalize with 'ß' |  69.5 ms (*) |    80 ms (+15%)
--------------------+--------------+----------------
Total               | 2.24 sec (*) | 2.71 sec (+21%)
--------------------+--------------+----------------

See bench.txt for the full output.