What causes the extreme slowness when changing from rails 3.2.12 to 3.2.13

This morning, I updated rails from 3.2.12 to 3.2.13, which resulted in a major delay in loading my views. This is from loading my home page:

Rails 3.2.12:
Completed 200 OK in 387ms (Views: 339.0ms | ActiveRecord: 27.1ms)

Rails 3.2.13:
Completed 200 OK in 4416ms (Views: 4361.2ms | ActiveRecord: 28.7ms)

The only difference between the two commits it the Rails version, which of course did also result in a lot of other gems being updated... This is the difference in the Gemfile.lock:

GEM
   remote: https://rubygems.org/
   specs:
-    actionmailer (3.2.12)
-      actionpack (= 3.2.12)
-      mail (~> 2.4.4)
-    actionpack (3.2.12)
-      activemodel (= 3.2.12)
-      activesupport (= 3.2.12)
+    actionmailer (3.2.13)
+      actionpack (= 3.2.13)
+      mail (~> 2.5.3)
+    actionpack (3.2.13)
+      activemodel (= 3.2.13)
+      activesupport (= 3.2.13)
       builder (~> 3.0.0)
       erubis (~> 2.7.0)
       journey (~> 1.0.4)
@@ -14,19 +14,19 @@ GEM
       rack-cache (~> 1.2)
       rack-test (~> 0.6.1)
       sprockets (~> 2.2.1)
-    activemodel (3.2.12)
-      activesupport (= 3.2.12)
+    activemodel (3.2.13)
+      activesupport (= 3.2.13)
       builder (~> 3.0.0)
-    activerecord (3.2.12)
-      activemodel (= 3.2.12)
-      activesupport (= 3.2.12)
+    activerecord (3.2.13)
+      activemodel (= 3.2.13)
+      activesupport (= 3.2.13)
       arel (~> 3.0.2)
       tzinfo (~> 0.3.29)
-    activeresource (3.2.12)
-      activemodel (= 3.2.12)
-      activesupport (= 3.2.12)
-    activesupport (3.2.12)
-      i18n (~> 0.6)
+    activeresource (3.2.13)
+      activemodel (= 3.2.13)
+      activesupport (= 3.2.13)
+    activesupport (3.2.13)
+      i18n (= 0.6.1)
       multi_json (~> 1.0)
     airbrake (3.1.7)
       activesupport
@@ -129,7 +129,7 @@ GEM
     hashr (0.0.22)
     hike (1.2.1)
     honeypot-captcha (0.0.2)
-    i18n (0.6.4)
+    i18n (0.6.1)
     journe