diff --git a/quartz/src/androidMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1Instance.android.kt b/quartz/src/androidMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1Instance.android.kt
index 789fe2404..725c8fedf 100644
--- a/quartz/src/androidMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1Instance.android.kt
+++ b/quartz/src/androidMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1Instance.android.kt
@@ -41,6 +41,14 @@ actual object Secp256k1Instance {
         privKey: ByteArray,
     ): ByteArray = secp256k1.signSchnorr(data, privKey, null)
 
+    // Native C lib always derives pubkey internally — ignore the cached pubkey.
+    actual fun signSchnorrWithXOnlyPubKey(
+        data: ByteArray,
+        privKey: ByteArray,
+        xOnlyPubKey: ByteArray,
+        nonce: ByteArray?,
+    ): ByteArray = secp256k1.signSchnorr(data, privKey, nonce)
+
     actual fun verifySchnorr(
         signature: ByteArray,
         hash: ByteArray,
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/crypto/Nip01Crypto.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/crypto/Nip01Crypto.kt
index 7022d353e..bf47cdff4 100644
--- a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/crypto/Nip01Crypto.kt
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/crypto/Nip01Crypto.kt
@@ -34,6 +34,18 @@ object Nip01Crypto {
         nonce: ByteArray? = RandomInstance.bytes(32),
     ): ByteArray = Secp256k1Instance.signSchnorr(data, privKey, nonce)
 
+    /**
+     * Fast sign with pre-computed x-only pubkey (32 bytes).
+     * Skips the expensive G multiplication to derive the pubkey (~20μs on Android).
+     * Use when the caller already has the pubkey (e.g., from KeyPair.pubKey).
+     */
+    fun signWithPubKey(
+        data: ByteArray,
+        privKey: ByteArray,
+        xOnlyPubKey: ByteArray,
+        nonce: ByteArray? = RandomInstance.bytes(32),
+    ): ByteArray = Secp256k1Instance.signSchnorrWithXOnlyPubKey(data, privKey, xOnlyPubKey, nonce)
+
     fun verify(
         signature: ByteArray,
         hash: ByteArray,
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1Instance.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1Instance.kt
index 594b025f9..49c435f26 100644
--- a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1Instance.kt
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1Instance.kt
@@ -36,6 +36,13 @@ expect object Secp256k1Instance {
         privKey: ByteArray,
     ): ByteArray
 
+    fun signSchnorrWithXOnlyPubKey(
+        data: ByteArray,
+        privKey: ByteArray,
+        xOnlyPubKey: ByteArray,
+        nonce: ByteArray? = RandomInstance.bytes(32),
+    ): ByteArray
+
     fun verifySchnorr(
         signature: ByteArray,
         hash: ByteArray,
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1InstanceOurs.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1InstanceOurs.kt
index 145de4675..324a83443 100644
--- a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1InstanceOurs.kt
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1InstanceOurs.kt
@@ -48,6 +48,14 @@ object Secp256k1InstanceOurs {
         nonce: ByteArray? = RandomInstance.bytes(32),
     ): ByteArray = Secp256k1.signSchnorrWithPubKey(data, privKey, compressedPubKey, nonce)
 
+    /** Fast signing with pre-computed x-only public key — zero-copy, zero-allocation. */
+    fun signSchnorrWithXOnlyPubKey(
+        data: ByteArray,
+        privKey: ByteArray,
+        xOnlyPubKey: ByteArray,
+        nonce: ByteArray? = RandomInstance.bytes(32),
+    ): ByteArray = Secp256k1.signSchnorrWithXOnlyPubKey(data, privKey, xOnlyPubKey, nonce)
+
     fun verifySchnorr(
         signature: ByteArray,
         hash: ByteArray,
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/Secp256k1.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/Secp256k1.kt
index e2f08dbf7..5ac2d9552 100644
--- a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/Secp256k1.kt
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/Secp256k1.kt
@@ -282,6 +282,28 @@ object Secp256k1 {
         return signSchnorrInternal(data, d0, xOnlyPub, hasEvenY, auxrand)
     }
 
+    /**
+     * Fast signing with a pre-computed x-only public key (32 bytes).
+     *
+     * BIP-340 public keys always have even y, so the y-parity is known (even = true).
+     * This avoids both the expensive G multiplication to derive the pubkey AND the
+     * 33→32 byte array copy that signSchnorrWithPubKey does internally.
+     *
+     * Use when the caller already has the 32-byte x-only pubkey (e.g., from KeyPair.pubKey).
+     */
+    fun signSchnorrWithXOnlyPubKey(
+        data: ByteArray,
+        seckey: ByteArray,
+        xOnlyPub: ByteArray,
+        auxrand: ByteArray?,
+    ): ByteArray {
+        require(seckey.size == 32 && xOnlyPub.size == 32)
+        val d0 = U256.fromBytes(seckey)
+        require(ScalarN.isValid(d0))
+        // BIP-340: x-only pubkeys always have even y
+        return signSchnorrInternal(data, d0, xOnlyPub, true, auxrand)
+    }
+
     /**
      * Internal signing implementation shared by both public overloads.
      * Performs: nonce derivation → R = k·G → challenge → s = k + e·d.
diff --git a/quartz/src/jvmMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1Instance.jvm.kt b/quartz/src/jvmMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1Instance.jvm.kt
index 789fe2404..725c8fedf 100644
--- a/quartz/src/jvmMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1Instance.jvm.kt
+++ b/quartz/src/jvmMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1Instance.jvm.kt
@@ -41,6 +41,14 @@ actual object Secp256k1Instance {
         privKey: ByteArray,
     ): ByteArray = secp256k1.signSchnorr(data, privKey, null)
 
+    // Native C lib always derives pubkey internally — ignore the cached pubkey.
+    actual fun signSchnorrWithXOnlyPubKey(
+        data: ByteArray,
+        privKey: ByteArray,
+        xOnlyPubKey: ByteArray,
+        nonce: ByteArray?,
+    ): ByteArray = secp256k1.signSchnorr(data, privKey, nonce)
+
     actual fun verifySchnorr(
         signature: ByteArray,
         hash: ByteArray,
diff --git a/quartz/src/nativeMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1Instance.native.kt b/quartz/src/nativeMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1Instance.native.kt
index 771c35916..eefd59b56 100644
--- a/quartz/src/nativeMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1Instance.native.kt
+++ b/quartz/src/nativeMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1Instance.native.kt
@@ -41,6 +41,13 @@ actual object Secp256k1Instance {
         privKey: ByteArray,
     ): ByteArray = secp256k1Ref.signSchnorr(data, privKey, null)
 
+    actual fun signSchnorrWithXOnlyPubKey(
+        data: ByteArray,
+        privKey: ByteArray,
+        xOnlyPubKey: ByteArray,
+        nonce: ByteArray?,
+    ): ByteArray = secp256k1Ref.signSchnorr(data, privKey, nonce)
+
     actual fun verifySchnorr(
         signature: ByteArray,
         hash: ByteArray,