diff --git a/activemodel/lib/active_model/type/big_integer.rb b/activemodel/lib/active_model/type/big_integer.rb
index ce173c8b1f69d..224cd62bc910e 100644
--- a/activemodel/lib/active_model/type/big_integer.rb
+++ b/activemodel/lib/active_model/type/big_integer.rb
@@ -23,32 +23,15 @@ module Type
     # All casting and serialization are performed in the same way as the
     # standard ActiveModel::Type::Integer type.
     class BigInteger < Integer
-      def serialize(value) # :nodoc:
-        case value
-        when ::Integer
-          # noop
-        when ::String
-          int = value.to_i
-          if int.zero? && value != "0"
-            return if non_numeric_string?(value)
-          end
-          value = int
-        else
-          value = super
-        end
-
-        value
-      end
-
-      def serialize_cast_value(value) # :nodoc:
-        value
-      end
-
       def serializable?(value, &_)
         true
       end
 
       private
+        def ensure_within_range!(value)
+          # noop
+        end
+
         def max_value
           ::Float::INFINITY
         end
diff --git a/activemodel/lib/active_model/type/integer.rb b/activemodel/lib/active_model/type/integer.rb
index b3c3f3eb3767c..613e643f60f99 100644
--- a/activemodel/lib/active_model/type/integer.rb
+++ b/activemodel/lib/active_model/type/integer.rb
@@ -77,18 +77,13 @@ def serialize(value)
           value = super
         end
 
-        if out_of_range?(value)
-          raise ActiveModel::RangeError, "#{value} is out of range for #{self.class} with limit #{_limit} bytes"
-        end
+        ensure_within_range!(value)
 
         value
       end
 
       def serialize_cast_value(value) # :nodoc:
-        if out_of_range?(value)
-          raise ActiveModel::RangeError, "#{value} is out of range for #{self.class} with limit #{_limit} bytes"
-        end
-
+        ensure_within_range!(value)
         value
       end
 
@@ -100,6 +95,13 @@ def serializable?(value)
       end
 
       private
+
+        def ensure_within_range!(value)
+          return unless out_of_range?(value)
+
+          raise ActiveModel::RangeError, "#{value} is out of range for #{self.class} with limit #{_limit} bytes"
+        end
+
         def out_of_range?(value)
           value && (@max <= value || @min > value)
         end
diff --git a/bench_big_int.rb b/bench_big_int.rb
new file mode 100644
index 0000000000000..cad164555c3fa
--- /dev/null
+++ b/bench_big_int.rb
@@ -0,0 +1,17 @@
+require 'benchmark/ips'
+require 'active_model'
+
+class TestModel
+  include ActiveModel::Model
+  include ActiveModel::Attributes
+
+  attribute :number, :big_integer
+end
+
+Benchmark.ips do |x|
+  model = TestModel.new
+  x.load! 'tmp/bigint_conversion_benchmark'
+
+  x.report("big_integer") { model.number = BigDecimal("9223372036854775807") }
+  x.save! 'tmp/bigint_conversion_benchmark'
+end