Merge pull request #531 from tituswinters/per-opt

A more numeric reason for targeted optimization.

CppCoreGuidelines.md

@@ -10006,7 +10006,13 @@ Optimizing a non-performance-critical part of a program has no effect on system
 ##### Note
 
 If your program spends most of its time waiting for the web or for a human, optimization of in-memory computation is probably useless.
-???
+
+Put another way: If your program spends 4% of its processing time doing
+computation A and 40% of its time doing computation B, a 50% improvement on A is
+only as impactful as a 5% improvement on B. (If you don't even know how much
+time is spent on A or B, see <a href="#Rper-reason">PER.1</a> and <a
+href="#Rper-Knuth">PER.2</a>.)
+
 
 ### <a name="Rper-simple"></a>PER.4: Don't assume that complicated code is necessarily faster than simple code