diff --git a/bin/sessionmgr/sessionmgr_impl.cc b/bin/sessionmgr/sessionmgr_impl.cc
index ad00f1b..5abe13c 100644
--- a/bin/sessionmgr/sessionmgr_impl.cc
+++ b/bin/sessionmgr/sessionmgr_impl.cc
@@ -195,10 +195,15 @@
   // shell (see how RunSessionShell() initializes session_shell_services_) to
   // lazily initialize the following services only once they are requested
   // for the first time.
-  finish_initialization_ = fit::defer<fit::closure>(
-      [this, session_shell_url = session_shell.url,
+  finish_initialization_ =
+      [this, called = false, session_shell_url = session_shell.url,
        ledger_token_manager = std::move(ledger_token_manager),
        story_shell = std::move(story_shell)]() mutable {
+        if (called) {
+          return;
+        }
+        called = true;
+
         InitializeLedger(std::move(ledger_token_manager));
         InitializeDeviceMap();
         InitializeMessageQueueManager();
@@ -209,7 +214,7 @@
         });
         InitializeClipboard();
         ReportEvent(ModularEvent::BOOTED_TO_SESSIONMGR);
-      });
+      };
 
   InitializeUser(std::move(account), std::move(agent_token_manager),
                  std::move(user_context));
@@ -703,7 +708,7 @@
         if (terminating_) {
           return;
         }
-        finish_initialization_.call();
+        finish_initialization_();
         session_shell_context_bindings_.AddBinding(this, std::move(request));
       });
 
@@ -713,7 +718,7 @@
         if (terminating_) {
           return;
         }
-        finish_initialization_.call();
+        finish_initialization_();
         session_shell_component_context_impl_->Connect(std::move(request));
       });
 
@@ -723,7 +728,7 @@
         if (terminating_) {
           return;
         }
-        finish_initialization_.call();
+        finish_initialization_();
         puppet_master_impl_->Connect(std::move(request));
       });
 
@@ -733,7 +738,7 @@
         if (terminating_) {
           return;
         }
-        finish_initialization_.call();
+        finish_initialization_();
         fuchsia::modular::ComponentScope component_scope;
         component_scope.set_global_scope(fuchsia::modular::GlobalScope());
         user_intelligence_provider_impl_->GetComponentIntelligenceServices(
diff --git a/bin/sessionmgr/sessionmgr_impl.h b/bin/sessionmgr/sessionmgr_impl.h
index 5cca15c..19cc922 100644
--- a/bin/sessionmgr/sessionmgr_impl.h
+++ b/bin/sessionmgr/sessionmgr_impl.h
@@ -21,7 +21,7 @@
 #include <lib/component/cpp/service_provider_impl.h>
 #include <lib/fidl/cpp/binding.h>
 #include <lib/fidl/cpp/interface_ptr.h>
-#include <lib/fit/defer.h>
+#include <lib/fit/function.h>
 #include <lib/fxl/macros.h>
 #include <zx/eventpair.h>
 
@@ -305,7 +305,10 @@
 
   OperationQueue operation_queue_;
 
-  fit::deferred_action<fit::closure> finish_initialization_;
+  // Part of Initialize() that is deferred until the first environment service
+  // request is received from the session shell, in order to accelerate the
+  // startup of session shell.
+  fit::function<void()> finish_initialization_{[]{}};
 
   // Set to |true| when sessionmgr starts its terminating sequence;  this flag
   // can be used to determine whether to reject vending FIDL services.