diff --git a/src/main/scala/com/raquo/laminar/codecs/CompositeCodec.scala b/src/main/scala/com/raquo/laminar/codecs/CompositeCodec.scala
index 610f4315..e917c899 100644
--- a/src/main/scala/com/raquo/laminar/codecs/CompositeCodec.scala
+++ b/src/main/scala/com/raquo/laminar/codecs/CompositeCodec.scala
@@ -1,7 +1,6 @@
 package com.raquo.laminar.codecs
 
-import com.raquo.ew.ewArray
-
+import scala.scalajs.js.|
 import scala.scalajs.js.JSStringOps._
 
 /** Such codecs are used for space-separated or comma-separated values, e.g. in the `cls` attr. */
@@ -14,6 +13,54 @@ class CompositeCodec(separator: String) extends Codec[Iterable[String], String]
   override def encode(scalaValue: Iterable[String]): String = {
     scalaValue.mkString(separator)
   }
+
+  /** Updates a composite value using this codec's decode and encode methods.
+    *
+    * @param domValue raw DOM value, `()` if the attribute is not set
+    * @param addItems must be normalized
+    */
+  def encodeUpdated(
+    domValue: String | Unit,
+    removeItems: List[String],
+    addItems: List[String]
+  ): String = {
+    encode(domValue.map(decode).getOrElse(Nil).filterNot(removeItems.contains) ++ addItems)
+  }
+}
+
+final private[laminar] class DefaultCompositeCodec(separator: String) extends CompositeCodec(separator) {
+
+  override def encodeUpdated(
+    domValue: String | Unit,
+    removeItems: List[String],
+    addItems: List[String]
+  ): String = {
+    var result = ""
+    var isFirst = true
+    val append = (item: String) => {
+      if (isFirst) {
+        result = item
+        isFirst = false
+      } else {
+        result = result + separator + item
+      }
+    }
+    domValue.foreach { value =>
+      if (value.nonEmpty) {
+        val items = value.jsSplit(separator)
+        var i = 0
+        while (i < items.length) {
+          val item = items(i)
+          if (item.nonEmpty && !removeItems.contains(item)) {
+            append(item)
+          }
+          i += 1
+        }
+      }
+    }
+    addItems.foreach(append)
+    result
+  }
 }
 
 object CompositeCodec {
@@ -29,8 +76,22 @@ object CompositeCodec {
   def normalize(items: String, separator: String): List[String] = {
     if (items.isEmpty) {
       Nil
+    } else if (items.indexOf(separator) == -1) {
+      // Fast path for the most common case: a single item, e.g. `cls := "active"`
+      items :: Nil
     } else {
-      items.jsSplit(separator).ew.filter(_.nonEmpty).asScalaJs.toList
+      val splitItems = items.jsSplit(separator)
+      // Iterate backwards to build the List by prepending, preserving order
+      var result: List[String] = Nil
+      var i = splitItems.length - 1
+      while (i >= 0) {
+        val item = splitItems(i)
+        if (item.nonEmpty) {
+          result = item :: result
+        }
+        i -= 1
+      }
+      result
     }
   }
 }
diff --git a/src/main/scala/com/raquo/laminar/keys/CompositeKey.scala b/src/main/scala/com/raquo/laminar/keys/CompositeKey.scala
index b700debb..70d3394e 100644
--- a/src/main/scala/com/raquo/laminar/keys/CompositeKey.scala
+++ b/src/main/scala/com/raquo/laminar/keys/CompositeKey.scala
@@ -2,7 +2,7 @@ package com.raquo.laminar.keys
 
 import com.raquo.airstream.core.Source
 import com.raquo.laminar.api.{MapValueMapper, StringOptionValueMapper, StringSeqValueMapper, StringValueMapper}
-import com.raquo.laminar.codecs.CompositeCodec
+import com.raquo.laminar.codecs.{CompositeCodec, DefaultCompositeCodec}
 import com.raquo.laminar.modifiers.{CompositeKeySetter, CompositeKeyUpdater}
 import com.raquo.laminar.nodes.ReactiveElement
 
@@ -32,7 +32,7 @@ abstract class CompositeKey[ //
 
   val separator: String
 
-  val codec: CompositeCodec = new CompositeCodec(separator)
+  val codec: CompositeCodec = new DefaultCompositeCodec(separator)
 
   def :=(items: String): CompositeKeySetter[Self, El] = {
     addStaticItems(StringValueMapper.toNormalizedList(items, separator))
diff --git a/src/main/scala/com/raquo/laminar/nodes/ReactiveElement.scala b/src/main/scala/com/raquo/laminar/nodes/ReactiveElement.scala
index 78d718a7..4fa477f2 100644
--- a/src/main/scala/com/raquo/laminar/nodes/ReactiveElement.scala
+++ b/src/main/scala/com/raquo/laminar/nodes/ReactiveElement.scala
@@ -127,13 +127,14 @@ with ParentNode[Ref] {
 
     val itemsToAdd = addItems.distinct
     val itemsToRemoveFromDom = removeItems.filterNot(itemHasAnotherReason)
-    val newItems = _compositeValues
-      .getOrElse(key, Nil)
+    val newItems = keyItemsWithReason
       .filterNot(t => removeItems.contains(t._1) && t._2 == reason) ++ itemsToAdd.map((_, reason))
 
-    val domValues = key.getRawDomValue(this).map(key.codec.decode).getOrElse(Nil)
-
-    val nextDomValues = domValues.filterNot(itemsToRemoveFromDom.contains) ++ itemsToAdd.filterNot(itemHasAnotherReason)
+    val nextDomValue = key.codec.encodeUpdated(
+      domValue = key.getRawDomValue(this),
+      removeItems = itemsToRemoveFromDom,
+      addItems = itemsToAdd.filterNot(itemHasAnotherReason)
+    )
 
     // 1. Update Laminar's internal structure
     _compositeValues = _compositeValues.updated(key, newItems)
@@ -142,7 +143,7 @@ with ParentNode[Ref] {
     // #Note this logic is compatible with third parties setting classes on Laminar elements
     //  using raw JS methods as long as they don't remove classes managed by Laminar or add
     //  classes that were also added by Laminar.
-    key.setRawDomValue(this, key.codec.encode(nextDomValues))
+    key.setRawDomValue(this, nextDomValue)
   }
 
   val tag: Tag[ReactiveElement[Ref]]
diff --git a/src/test/scala/com/raquo/laminar/tests/CompositeKeySpec.scala b/src/test/scala/com/raquo/laminar/tests/CompositeKeySpec.scala
index 17ce1a4b..d6cb1c4f 100644
--- a/src/test/scala/com/raquo/laminar/tests/CompositeKeySpec.scala
+++ b/src/test/scala/com/raquo/laminar/tests/CompositeKeySpec.scala
@@ -1,10 +1,39 @@
 package com.raquo.laminar.tests
 
 import com.raquo.laminar.api.L._
+import com.raquo.laminar.codecs.CompositeCodec
+import com.raquo.laminar.keys.CompositeAttr
+import com.raquo.laminar.nodes.ReactiveElement
 import com.raquo.laminar.utils.UnitSpec
 
 class CompositeKeySpec extends UnitSpec {
 
+  it("custom composite codec uses its encode and decode overrides") {
+    val customKey = new CompositeAttr[ReactiveElement.Base]("data-custom", " ") {
+      override val codec: CompositeCodec = new CompositeCodec(" ") {
+        override def decode(domValue: String): List[String] = {
+          super.decode(domValue.stripPrefix("(").stripSuffix(")"))
+        }
+
+        override def encode(scalaValue: Iterable[String]): String = {
+          "(" + super.encode(scalaValue) + ")"
+        }
+      }
+    }
+    val item = Var("bar")
+    val el = div(customKey := "foo", customKey <-- item)
+    mount(el)
+
+    withClue("Initial binding:") {
+      assert(el.ref.getAttribute("data-custom") == "(foo bar)")
+    }
+
+    withClue("Reactive update:") {
+      item.set("baz")
+      assert(el.ref.getAttribute("data-custom") == "(foo baz)")
+    }
+  }
+
   it("cls - simple static modifiers") {
     val el = div(className := "foo")
     mount(el)