Report some benchmark results.

pep-0393.txt

@@ -266,6 +266,11 @@ string sizes and platforms.
 |8      | 56      | 80   | 72     |  96   | 56     | 88    |
 +-------+---------+------+--------+-------+--------+-------+
 
+The runtime effect is significantly affected by the API being
+used. After porting the relevant pieces of code to the new API,
+the iobench, stringbench, and json benchmarks see typically
+slowdowns of 1% to 30%; for specific benchmarks, speedups may
+happen as may happen significantly larger slowdowns.
 
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