diff --git a/middleman-core/lib/middleman-core/sources.rb b/middleman-core/lib/middleman-core/sources.rb
index ca58f656..cd3eab17 100644
--- a/middleman-core/lib/middleman-core/sources.rb
+++ b/middleman-core/lib/middleman-core/sources.rb
@@ -1,4 +1,5 @@
 require 'middleman-core/contracts'
+require 'backports/2.0.0/enumerable/lazy'
 
 module Middleman
   # The standard "record" that contains information about a file on disk.
@@ -168,6 +169,7 @@ module Middleman
     Contract Symbol, String, Maybe[Bool] => Maybe[SourceFile]
     def find(type, path, glob=false)
       watchers
+          .lazy
           .select { |d| d.type == type }
           .map { |d| d.find(path, glob) }
           .reject { |d| d.nil? }
@@ -182,6 +184,7 @@ module Middleman
     Contract Symbol, String => Bool
     def exists?(type, path)
       watchers
+          .lazy
           .select { |d| d.type == type }
           .any? { |d| d.exists?(path) }
     end
diff --git a/middleman-core/lib/middleman-core/sources/source_watcher.rb b/middleman-core/lib/middleman-core/sources/source_watcher.rb
index 9fb05475..bfc0878f 100644
--- a/middleman-core/lib/middleman-core/sources/source_watcher.rb
+++ b/middleman-core/lib/middleman-core/sources/source_watcher.rb
@@ -1,7 +1,7 @@
 # Watcher Library
 require 'listen'
-
 require 'middleman-core/contracts'
+require 'backports/2.0.0/enumerable/lazy'
 
 module Middleman
   # The default source watcher implementation. Watches a directory on disk
@@ -211,25 +211,31 @@ module Middleman
     Contract ArrayOf[Pathname], ArrayOf[Pathname] => Any
     def update(updated_paths, removed_paths)
       valid_updates = updated_paths
+          .lazy
           .map(&method(:path_to_source_file))
           .select(&method(:valid?))
-
-      valid_updates.each do |f|
-        add_file_to_cache(f)
-        logger.debug "== Change (#{f[:type]}): #{f[:relative_path]}"
-      end
+          .to_a
+          .each do |f|
+            add_file_to_cache(f)
+            logger.debug "== Change (#{f[:type]}): #{f[:relative_path]}"
+          end
 
       valid_removes = removed_paths
+          .lazy
           .select(&@files.method(:key?))
           .map(&@files.method(:[]))
           .select(&method(:valid?))
+          .to_a
+          .each do |f|
+            remove_file_from_cache(f)
+            logger.debug "== Deletion (#{f[:type]}): #{f[:relative_path]}"
+          end
 
-      valid_removes.each do |f|
-        remove_file_from_cache(f)
-        logger.debug "== Deletion (#{f[:type]}): #{f[:relative_path]}"
-      end
-
-      run_callbacks(@on_change_callbacks, valid_updates, valid_removes) unless valid_updates.empty? && valid_removes.empty?
+      run_callbacks(
+        @on_change_callbacks,
+        valid_updates,
+        valid_removes
+      ) unless valid_updates.empty? && valid_removes.empty?
     end
 
     def add_file_to_cache(f)
@@ -260,7 +266,7 @@ module Middleman
     Contract Pathname => IsA['Middleman::SourceFile']
     def path_to_source_file(path)
       ::Middleman::SourceFile.new(
-        path.relative_path_from(@directory), path, @directory, @type)
+          path.relative_path_from(@directory), path, @directory, @type)
     end
 
     # Notify callbacks for a file given an array of callbacks
diff --git a/middleman-core/middleman-core.gemspec b/middleman-core/middleman-core.gemspec
index f8735917..397928dc 100644
--- a/middleman-core/middleman-core.gemspec
+++ b/middleman-core/middleman-core.gemspec
@@ -20,6 +20,7 @@ Gem::Specification.new do |s|
 
   # Core
   s.add_dependency('bundler', ['~> 1.1'])
+  s.add_dependency('backports', ['~> 3.6'])
   s.add_dependency('rack', ['>= 1.4.5', '< 2.0'])
   s.add_dependency('tilt', ['~> 1.4.1'])
   s.add_dependency('erubis')