_results

$ ruby -Ilib benchmark.rb
Warming up --------------------------------------
  require whitespace    18.000  i/100ms
don't require whitespace
                        19.000  i/100ms
Calculating -------------------------------------
  require whitespace    195.417  (± 1.5%) i/s -    990.000  in   5.067471s
don't require whitespace
                        192.750  (± 2.6%) i/s -    969.000  in   5.030580s

Comparison:
  require whitespace:      195.4 i/s
don't require whitespace:      192.7 i/s - same-ish: difference falls within error

benchmark.rb

require 'benchmark/ips'
require 'kramdown'

Benchmark.ips do |x|

  x.report("require whitespace") do
    ENV["REQUIRE_WHITESPACE"] = "true"
    Kramdown::Document.new("## 1#{" "*20000}2")
  end

  x.report("don't require whitespace") do
    ENV.delete("REQUIRE_WHITESPACE")
    Kramdown::Document.new("## 1#{" "*20000}2")
  end

  x.compare!
end

header.rb.diff

diff --git a/lib/kramdown/parser/kramdown/header.rb b/lib/kramdown/parser/kramdown/header.rb
index 700597e..f680b57 100644
--- a/lib/kramdown/parser/kramdown/header.rb
+++ b/lib/kramdown/parser/kramdown/header.rb
@@ -31,7 +31,11 @@ def parse_setext_header
       def parse_atx_header
         return false unless after_block_boundary?
         text, id = parse_header_contents
-        text.sub!(/(?<!\\)#+\z/, '') && text.rstrip!
+        if ENV['REQUIRE_WHITESPACE']
+          text.sub!(/[\t ]#+\z/, '') && text.rstrip!
+        else
+          text.sub!(/(?<!\\)#+\z/, '') && text.rstrip!
+        end
         return false if text.empty?
         add_header(@src["level"].length, text, id)
         true