Update Rper-simple example comment (#1508)

The example that is 'intended' to be faster is actually faster on clang 8 /w -O3, and marginally slower than the 'simple' example on GCC 9.1 /w -O3
http://quick-bench.com/DrXvcvhjS1zDmMkgxrzIjbMByYA
http://quick-bench.com/03VXPgDSxiMAmZMItiEA0iwTJgM

CppCoreGuidelines.md

@@ -13387,7 +13387,7 @@ Simple code can be very fast. Optimizers sometimes do marvels with simple code
 
 ##### Example, bad
 
-    // intended to be faster, but is actually slower
+    // intended to be faster, but is often slower
 
     vector<uint8_t> v(100000);