Why are my tests slow after upgrading rails from 3.1.0 to 3.2.0?

I followed these upgrade instructions: http://railscasts.com/episodes/318-upgrading-to-rails-3-2

Here are my three small upgrade changes:

(1) Gemfile

-gem 'rails', '3.1.0'
+gem 'rails', '3.2.0'

-gem 'rack', '1.3.3'
+#gem 'rack', '1.3.3'

 group :assets do
-  gem 'sass-rails', '  ~> 3.1.0'
-  gem 'coffee-rails', '~> 3.1.0'
-  gem 'uglifier'
+  gem 'sass-rails', '  ~> 3.2.3'
+  gem 'coffee-rails', '~> 3.2.1'
+  gem 'uglifier', '     >=1.0.3'
   gem 'asset_sync'
 end

(2) config/environments/development.rb

+  config.active_record.mass_assignment_sanitizer = :strict
+  config.active_record.auto_explain_threshold_in_seconds = 0.5

(3) config/environments/test.rb

-  config.assets.allow_debugging = true
+  config.active_record.mass_assignment_sanitizer = :strict

Prior to the upgrade, my tests looked as follows (well under a second each):

...
StockroomTest:
     PASS stockroom must have a name (0.03s) 
     PASS stockroom name must be unique (0.01s) 
     PASS stockroom with name is valid (0.00s) 
...
Finished in 1.604118 seconds.
29 tests, 90 assertions, 0 failures, 0 errors, 0 skips
...
StockroomsControllerTest:
     PASS should create stockroom (0.04s)
     PASS should destroy stockroom (0.02s)
     PASS should get edit (0.14s)
     PASS should get index (0.11s)
     PASS should get new (0.03s)
     PASS should not destroy stockroom (0.04s)
     PASS should show stockroom (0.13s)
     PASS should update stockroom (0.02s)
...
Finished in 12.572911 seconds.
115 tests, 166 assertions, 0 failures, 0 errors, 0 skips
...
MiscellaneousTest:
     PASS get campaigns#index should redirect to newsletters#index (1.83s)
     PASS get /campaigns should redirect to / when logged out (0.06s)
Finished