diff --git a/src/main/java/com/gregor/jprototerm/Main.java b/src/main/java/com/gregor/jprototerm/Main.java
index 569c8df..6330ce0 100644
--- a/src/main/java/com/gregor/jprototerm/Main.java
+++ b/src/main/java/com/gregor/jprototerm/Main.java
@@ -31,10 +31,17 @@ public final class Main extends Application {
 
     @Override
     public void start(Stage stage) {
+        // First mark: time from JVM start through JavaFX toolkit + GL pipeline init (start() is the
+        // first app code the toolkit runs). Usually the dominant slice of cold startup.
+        StartupTiming.mark("toolkit ready (start)");
         config = AppConfig.load();
+        StartupTiming.mark("config loaded");
 
         metrics = new TerminalMetrics(config.fontFamily(), config.fontSize());
+        StartupTiming.mark("fonts loaded");
         compositor = new Compositor(config, metrics);
+        // Includes the first Ghostty.open (native dlopen) and the first pty spawn.
+        StartupTiming.mark("compositor ready");
         // When the last pane closes — whether via the close-pane key or because a pane's process
         // exited on its own — tear down and quit.
         compositor.setOnEmpty(() -> {
@@ -60,6 +67,7 @@ public final class Main extends Application {
             @Override
             public void handle(long now) {
                 compositor.render();
+                StartupTiming.firstFrame();
             }
         }.start();
 
@@ -72,6 +80,7 @@ public final class Main extends Application {
         // to honour, so place it on the screen under the mouse pointer instead.
         centreOnActiveScreen(stage, config.windowWidth(), config.windowHeight());
         stage.show();
+        StartupTiming.mark("stage shown");
         // Ask the window manager to raise and focus the new window so the user can type right
         // away; the canvas requestFocus() below only routes events within the scene.
         stage.toFront();
diff --git a/src/main/java/com/gregor/jprototerm/StartupTiming.java b/src/main/java/com/gregor/jprototerm/StartupTiming.java
new file mode 100644
index 0000000..d5fe286
--- /dev/null
+++ b/src/main/java/com/gregor/jprototerm/StartupTiming.java
@@ -0,0 +1,48 @@
+package com.gregor.jprototerm;
+
+import java.lang.management.ManagementFactory;
+
+/**
+ * Opt-in startup phase timing, enabled with {@code -Djprototerm.timing=true} (e.g. via
+ * {@code JAVA_TOOL_OPTIONS}); otherwise every method is a cheap no-op and prints nothing.
+ *
+ * <p>Each {@link #mark(String)} prints one line to stderr with the time since the previous mark and
+ * the total since JVM start, so a cold launch breaks down into its phases — toolkit/GL init vs
+ * config load vs font loading vs first frame. The anchor is the JVM's own start time (the closest
+ * proxy we have to "process start"), so the first mark includes JVM bootstrap and JavaFX toolkit
+ * init, which is usually the dominant cost.
+ */
+final class StartupTiming {
+    private static final boolean ENABLED = Boolean.getBoolean("jprototerm.timing");
+    // Epoch millis; getStartTime() is the JVM's start, the earliest timestamp we can anchor to.
+    private static final long JVM_START_MILLIS = ManagementFactory.getRuntimeMXBean().getStartTime();
+    private static long lastMillis = -1;
+    private static boolean firstFrameSeen;
+
+    private StartupTiming() {
+    }
+
+    /** Records a phase boundary, printing the delta since the previous mark and since JVM start. */
+    static void mark(String phase) {
+        if (!ENABLED) {
+            return;
+        }
+        long now = System.currentTimeMillis();
+        long sinceStart = now - JVM_START_MILLIS;
+        long sinceLast = lastMillis < 0 ? sinceStart : now - lastMillis;
+        lastMillis = now;
+        System.err.printf("[timing] %-22s +%5d ms   (%5d ms since JVM start)%n", phase, sinceLast, sinceStart);
+    }
+
+    /**
+     * Records the first rendered frame exactly once, then becomes a no-op. Safe and cheap to call
+     * from the render loop every frame (it only ever touches FX-thread state).
+     */
+    static void firstFrame() {
+        if (!ENABLED || firstFrameSeen) {
+            return;
+        }
+        firstFrameSeen = true;
+        mark("first frame");
+    }
+}