Add String#start_with? vs String#[].==

README.md

@@ -884,6 +884,22 @@ Comparison:
            String#=~:   891124.1 i/s - 3.30x slower
 ```
 
+##### `String#start_with?` vs `String#[].==` [code](code/string/start_with-vs-substring-==.rb)
+
+```
+ruby 2.2.4p230 (2015-12-16 revision 53155) [i386-cygwin]
+
+Calculating -------------------------------------
+  String#start_with?     54.979k (± 0.4%) i/s -    279.450k in   5.082987s
+    String#[0, n] ==     31.488k (± 0.2%) i/s -    160.166k in   5.086555s
+   String#[0...n] ==     20.715k (± 0.3%) i/s -    105.300k in   5.083218s
+
+Comparison:
+  String#start_with?:    54978.5 i/s
+    String#[0, n] ==:    31488.3 i/s - 1.75x slower
+   String#[0...n] ==:    20715.5 i/s - 2.65x slower
+```
+
 ##### `Regexp#===` vs `String#match` vs `String#=~` [code ](code/string/===-vs-=~-vs-match.rb)
 
 > :warning: <br>

code/string/start_with-vs-substring-==.rb

@@ -0,0 +1,24 @@
+require 'benchmark/ips'
+
+PREFIX = '_'
+STRINGS = (0..99).map{|n| "#{PREFIX if n.odd?}#{n}" }
+
+def fast
+  STRINGS.each{|s| s.start_with?(PREFIX) }
+end
+
+def slow_length
+  STRINGS.each{|s| s[0, PREFIX.length] == PREFIX }
+end
+
+def slow_range
+  STRINGS.each{|s| s[0...PREFIX.length] == PREFIX }
+end
+
+
+Benchmark.ips do |x|
+  x.report('String#start_with?') { fast }
+  x.report('String#[0, n] ==')   { slow_length }
+  x.report('String#[0...n] ==')  { slow_range }
+  x.compare!
+end