diff --git a/lib/dead_end.rb b/lib/dead_end.rb
index f93a276..566e7d0 100644
--- a/lib/dead_end.rb
+++ b/lib/dead_end.rb
@@ -147,6 +147,7 @@ def self.valid?(source)
 
 require_relative "dead_end/lex_all"
 require_relative "dead_end/block_expand"
+require_relative "dead_end/insertion_sort"
 require_relative "dead_end/around_block_scan"
 require_relative "dead_end/ripper_errors"
 require_relative "dead_end/display_invalid_blocks"
diff --git a/lib/dead_end/code_frontier.rb b/lib/dead_end/code_frontier.rb
index e068a0b..d33f154 100644
--- a/lib/dead_end/code_frontier.rb
+++ b/lib/dead_end/code_frontier.rb
@@ -52,14 +52,14 @@ module DeadEnd
   class CodeFrontier
     def initialize(code_lines:)
       @code_lines = code_lines
-      @frontier = []
+      @frontier = InsertionSort.new
       @unvisited_lines = @code_lines.sort_by(&:indent_index)
       @has_run = false
       @check_next = true
     end
 
     def count
-      @frontier.count
+      @frontier.to_a.length
     end
 
     # Performance optimization
@@ -89,7 +89,7 @@ def count
     def holds_all_syntax_errors?(block_array = @frontier, can_cache: true)
       return false if can_cache && can_skip_check?
 
-      without_lines = block_array.flat_map do |block|
+      without_lines = block_array.to_a.flat_map do |block|
         block.lines
       end
 
@@ -101,7 +101,7 @@ def holds_all_syntax_errors?(block_array = @frontier, can_cache: true)
 
     # Returns a code block with the largest indentation possible
     def pop
-      @frontier.pop
+      @frontier.to_a.pop
     end
 
     def next_indent_line
@@ -109,15 +109,15 @@ def next_indent_line
     end
 
     def expand?
-      return false if @frontier.empty?
-      return true if @unvisited_lines.empty?
+      return false if @frontier.to_a.empty?
+      return true if @unvisited_lines.to_a.empty?
 
-      frontier_indent = @frontier.last.current_indent
+      frontier_indent = @frontier.to_a.last.current_indent
       unvisited_indent = next_indent_line.indent
 
       if ENV["DEBUG"]
         puts "```"
-        puts @frontier.last.to_s
+        puts @frontier.to_a.last.to_s
         puts "```"
         puts "  @frontier indent:  #{frontier_indent}"
         puts "  @unvisited indent: #{unvisited_indent}"
@@ -141,13 +141,13 @@ def <<(block)
       register_indent_block(block)
 
       # Make sure we don't double expand, if a code block fully engulfs another code block, keep the bigger one
-      @frontier.reject! { |b|
+      @frontier.to_a.reject! { |b|
         b.starts_at >= block.starts_at && b.ends_at <= block.ends_at
       }
 
       @check_next = true if block.invalid?
       @frontier << block
-      @frontier.sort!
+      # @frontier.sort!
 
       self
     end
@@ -167,7 +167,7 @@ def self.combination(array)
     # Given that we know our syntax error exists somewhere in our frontier, we want to find
     # the smallest possible set of blocks that contain all the syntax errors
     def detect_invalid_blocks
-      self.class.combination(@frontier.select(&:invalid?)).detect do |block_array|
+      self.class.combination(@frontier.to_a.select(&:invalid?)).detect do |block_array|
         holds_all_syntax_errors?(block_array, can_cache: false)
       end || []
     end
diff --git a/lib/dead_end/insertion_sort.rb b/lib/dead_end/insertion_sort.rb
new file mode 100644
index 0000000..6dbadf8
--- /dev/null
+++ b/lib/dead_end/insertion_sort.rb
@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+module DeadEnd
+  # Sort elements on insert
+  #
+  # Instead of constantly calling `sort!`, put
+  # the element where it belongs the first time
+  # around
+  #
+  # Example:
+  #
+  #   sorted = InsertionSort.new
+  #   sorted << 33
+  #   sorted << 44
+  #   sorted << 1
+  #   puts sorted.to_a
+  #   # => [1, 44, 33]
+  #
+  class InsertionSort
+    def initialize
+      @array = []
+    end
+
+    def <<(value)
+      insert_in = @array.length
+      @array.each.with_index do |existing, index|
+        case value <=> existing
+        when -1
+          insert_in = index
+          break
+        when 0
+          insert_in = index
+          break
+        when 1
+          # Keep going
+        end
+      end
+
+      @array.insert(insert_in, value)
+    end
+
+    def to_a
+      @array
+    end
+  end
+end
diff --git a/spec/unit/insertion_sort_spec.rb b/spec/unit/insertion_sort_spec.rb
new file mode 100644
index 0000000..e2987ae
--- /dev/null
+++ b/spec/unit/insertion_sort_spec.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+require_relative "../spec_helper"
+
+module DeadEnd
+  class CurrentIndex
+    attr_reader :current_indent
+
+    def initialize(value)
+      @current_indent = value
+    end
+
+    def <=>(other)
+      @current_indent <=> other.current_indent
+    end
+  end
+
+  RSpec.describe CodeFrontier do
+    it "works manually" do
+      frontier = InsertionSort.new
+      frontier << CurrentIndex.new(0)
+      frontier << CurrentIndex.new(1)
+
+      expect(frontier.to_a.map(&:current_indent)).to eq([0, 1])
+
+      frontier << CurrentIndex.new(1)
+      expect(frontier.to_a.map(&:current_indent)).to eq([0, 1, 1])
+
+      frontier << CurrentIndex.new(0)
+      expect(frontier.to_a.map(&:current_indent)).to eq([0, 0, 1, 1])
+
+      frontier << CurrentIndex.new(10)
+      expect(frontier.to_a.map(&:current_indent)).to eq([0, 0, 1, 1, 10])
+
+      frontier << CurrentIndex.new(2)
+      expect(frontier.to_a.map(&:current_indent)).to eq([0, 0, 1, 1, 2, 10])
+    end
+
+    it "handles lots of values" do
+      frontier = InsertionSort.new
+      values = [18, 18, 0, 18, 0, 18, 18, 18, 18, 16, 18, 8, 18, 8, 8, 8, 16, 6, 0, 0, 16, 16, 4, 14, 14, 12, 12, 12, 10, 12, 12, 12, 12, 8, 10, 10, 8, 8, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 8, 10, 6, 6, 6, 6, 6, 6, 8, 10, 8, 8, 10, 8, 10, 8, 10, 8, 6, 8, 8, 6, 8, 6, 6, 8, 0, 8, 0, 0, 8, 8, 0, 8, 0, 8, 8, 0, 8, 8, 8, 0, 8, 0, 8, 8, 8, 8, 8, 8, 8, 8, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 8, 8, 8, 8, 6, 8, 6, 6, 6, 6, 8, 6, 8, 6, 6, 4, 4, 6, 6, 4, 6, 4, 6, 6, 4, 6, 4, 4, 6, 6, 6, 6, 4, 4, 4, 2, 4, 4, 4, 4, 4, 4, 6, 6, 0, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 0, 0, 6, 6, 2]
+      values.each do |v|
+        frontier << CurrentIndex.new(v)
+      end
+
+      expect(frontier.to_a.map(&:current_indent)).to eq(values.sort)
+    end
+  end
+end