diff --git a/app/services/zxcvbn_service.rb b/app/services/zxcvbn_service.rb
index 0b11d4135..a6097ccc0 100644
--- a/app/services/zxcvbn_service.rb
+++ b/app/services/zxcvbn_service.rb
@@ -1,4 +1,32 @@
 class ZxcvbnService
+  @tester_mutex = Mutex.new
+
+  class << self
+    # Returns an Zxcvbn instance cached between classes instances and between threads.
+    #
+    # The tester weights ~20 Mo, and we'd like to save some memory – so rather
+    # that storing it in a per-thread accessor, we prefer to use a mutex
+    # to cache it between threads.
+    def tester
+      @tester_mutex.synchronize do
+        @tester ||= build_tester
+      end
+    end
+
+    private
+
+    # Returns a fully initializer tester from the on-disk dictionary.
+    #
+    # This is slow: loading and parsing the dictionary may take around 1s.
+    def build_tester
+      dictionaries = YAML.safe_load(File.read(Rails.root.join("config", "initializers", "zxcvbn_dictionnaries.yaml")))
+
+      tester = Zxcvbn::Tester.new
+      tester.add_word_lists(dictionaries)
+      tester
+    end
+  end
+
   def initialize(password)
     @password = password
   end
@@ -18,6 +46,6 @@ class ZxcvbnService
   private
 
   def compute_zxcvbn
-    Zxcvbn.test(@password, [], ZXCVBN_DICTIONNARIES)
+    self.class.tester.test(@password)
   end
 end
diff --git a/config/initializers/zxcvbn.rb b/config/initializers/zxcvbn.rb
deleted file mode 100644
index 416e80192..000000000
--- a/config/initializers/zxcvbn.rb
+++ /dev/null
@@ -1 +0,0 @@
-ZXCVBN_DICTIONNARIES = YAML.safe_load(File.read(Rails.root.join("config", "initializers", "zxcvbn_dictionnaries.yaml")))
diff --git a/spec/services/zxcvbn_service_spec.rb b/spec/services/zxcvbn_service_spec.rb
new file mode 100644
index 000000000..337da8c35
--- /dev/null
+++ b/spec/services/zxcvbn_service_spec.rb
@@ -0,0 +1,47 @@
+describe ZxcvbnService do
+  let(:password) { 'medium-strength-password' }
+  subject(:service) { ZxcvbnService.new(password) }
+
+  describe '#score' do
+    it 'returns the password complexity score' do
+      expect(service.score).to eq 3
+    end
+  end
+
+  describe '#complexity' do
+    it 'returns the password score, vulnerability and length' do
+      expect(service.complexity).to eq [3, 'medium, strength, password', 24]
+    end
+  end
+
+  describe 'caching' do
+    it 'lazily caches the tester between calls and instances' do
+      allow(Zxcvbn::Tester).to receive(:new).and_call_original
+      allow(YAML).to receive(:safe_load).and_call_original
+
+      first_service = ZxcvbnService.new('some-password')
+      first_service.score
+      first_service.complexity
+      other_service = ZxcvbnService.new('other-password')
+      other_service.score
+      other_service.complexity
+
+      expect(Zxcvbn::Tester).to have_received(:new).at_most(:once)
+      expect(YAML).to have_received(:safe_load).at_most(:once)
+    end
+
+    it 'lazily caches the tester between threads' do
+      allow(Zxcvbn::Tester).to receive(:new).and_call_original
+
+      threads = 1.upto(4).map do
+        Thread.new do
+          ZxcvbnService.new(password).score
+        end
+      end.map(&:join)
+
+      scores = threads.map(&:value)
+      expect(scores).to eq([3, 3, 3, 3])
+      expect(Zxcvbn::Tester).to have_received(:new).at_most(:once)
+    end
+  end
+end