diff --git a/main/src/main/scala/sbt/internal/IncrementalTest.scala b/main/src/main/scala/sbt/internal/IncrementalTest.scala
index 2a20d9981..6105e1349 100644
--- a/main/src/main/scala/sbt/internal/IncrementalTest.scala
+++ b/main/src/main/scala/sbt/internal/IncrementalTest.scala
@@ -59,9 +59,10 @@ object IncrementalTest:
     val rds = Keys.resourceDigests.value
     val extra = Keys.extraTestDigests.value
     val stamper = ClassStamper(cp, converter)
+    val testDigestExtra = extra ++ rds ++ opts
     // TODO: Potentially do something about JUnit 5 and others which might not use class name
     Map((testNames.flatMap: name =>
-      stamper.transitiveStamp(name, extra ++ rds ++ opts, s.log) match
+      stamper.transitiveStamp(name, testDigestExtra, s.log) match
         case Some(ts) => Seq(name -> ts)
         case None     => Nil
     )*)
@@ -174,11 +175,35 @@ class ClassStamper private[sbt] (
       converter,
     )
 
-  private val stamps = mutable.Map.empty[String, Set[Digest]]
+  // Leaf digests (class bytecode hashes + library file digests) are interned to dense
+  // ints so transitive digest sets can be held as bit sets: union is a word-parallel OR
+  // and each member costs one bit instead of a 32-byte Digest.
+  private val digestIds = mutable.HashMap.empty[Digest, Int]
+  private val digestList = mutable.ArrayBuffer.empty[Digest]
+  private def idOf(d: Digest): Int =
+    digestIds.getOrElseUpdate(d, { val i = digestList.size; digestList += d; i })
+
+  private val stamps = mutable.Map.empty[String, mutable.BitSet]
+  // Memoizes the full transitive digest set per class name (excluding extraHashes), so the
+  // re-entrant external-dep walk isn't recomputed for every reference. Mapping bits back to
+  // digests and sorting is deferred to the root (`transitiveStamp`); intermediate results
+  // are only ever OR-ed into another bit set, where order and identity are irrelevant.
+  private val transitiveCache = mutable.Map.empty[String, mutable.BitSet]
   // Cached so by-name `analyses0` is only evaluated once
   private lazy val analyses = analyses0
-  private val stampVf: VirtualFileRef => Digest =
-    CacheImplicits.virtualFileRefToDigest(_)(converter)
+  // Index of binary class name -> analyses that produce it, so a stamp can dispatch
+  // straight to its owning analyses instead of scanning every analysis on the classpath.
+  private lazy val analysesByProduct: Map[String, Seq[Analysis]] =
+    val acc = mutable.HashMap.empty[String, mutable.ListBuffer[Analysis]]
+    analyses.foreach: a =>
+      a.relations.productClassName._2s.foreach: bin =>
+        acc.getOrElseUpdate(bin, mutable.ListBuffer.empty) += a
+    acc.iterator.map((k, v) => k -> v.toSeq).toMap
+  // Memoized: virtualFileRefToDigest does a filesystem stat per call, and the same
+  // library ref is referenced by many classes.
+  private val vfDigests = mutable.Map.empty[VirtualFileRef, Digest]
+  private def stampVf(vf: VirtualFileRef): Digest =
+    vfDigests.getOrElseUpdate(vf, CacheImplicits.virtualFileRefToDigest(vf)(converter))
 
   /**
    * Given a classpath and a class name, this tries to create a SHA-256 digest.
@@ -190,22 +215,33 @@ class ClassStamper private[sbt] (
       extraHashes: Seq[Digest],
       log: Logger,
   ): Option[Digest] =
-    val digests = transitiveStamps(javaClassName, extraHashes, log)
+    val digests = sortedDigests(transitiveStamps(javaClassName, log)) ++ extraHashes
     if digests.nonEmpty then Some(Digest.sha256Hash(digests*))
     else None
 
+  // Map a bit set back to its digests
+  private def sortedDigests(bits: mutable.BitSet): Seq[Digest] =
+    val buf = mutable.ArrayBuffer.empty[Digest]
+    bits.foreach(i => buf += digestList(i))
+    buf.sortInPlace()
+    buf.toSeq
+
   private def transitiveStamps(
       javaClassName: String,
-      extraHashes: Seq[Digest],
       log: Logger,
-  ): Seq[Digest] =
-    val builder = Set.newBuilder[Digest]
-    analyses.foreach(internalStamp(builder, javaClassName, _, mutable.Set.empty, log))
-    val digests = builder.result().toSeq.sorted
-    digests ++ extraHashes
+  ): mutable.BitSet =
+    transitiveCache.getOrElseUpdate(
+      javaClassName, {
+        val builder = mutable.BitSet.empty
+        analysesByProduct
+          .getOrElse(javaClassName, Nil)
+          .foreach(internalStamp(builder, javaClassName, _, mutable.Set.empty, log))
+        builder
+      }
+    )
 
   private def internalStamp(
-      builder: mutable.Builder[Digest, Set[Digest]],
+      builder: mutable.BitSet,
       javaClassName: String,
       analysis: Analysis,
       alreadySeen: mutable.Set[String],
@@ -215,8 +251,8 @@ class ClassStamper private[sbt] (
 
     // log.debug(s"test: internalStamp($javaClassName)")
     def internalStamp0(className: String): Unit =
-      // Use a new builder so we can cache the result in `stamps`
-      val newBuilder = Set.newBuilder[Digest]
+      // Use a new bit set so we can cache the result in `stamps`
+      val newBuilder = mutable.BitSet.empty
 
       // Zinc doesn't fully track the transitive dependencies
       relations
@@ -227,32 +263,27 @@ class ClassStamper private[sbt] (
       relations
         .externalDeps(className)
         .foreach: libClassName =>
-          newBuilder ++= transitiveStamps(libClassName, Nil, log)
-      relations
-        .externalDeps(className)
-        .foreach: libClassName =>
+          newBuilder |= transitiveStamps(libClassName, log)
           relations.libraryClassName
             .reverse(libClassName)
             .foreach: vf =>
-              newBuilder += stampVf(vf)
+              newBuilder += idOf(stampVf(vf))
       analysis.apis.internal
         .get(className)
-        .toSet
         .foreach: analyzed =>
-          newBuilder += Digest.dummy(
-            37 * (17 + analyzed.transitiveBytecodeHash) + analyzed.bytecodeHash
+          newBuilder += idOf(
+            Digest.dummy(37 * (17 + analyzed.transitiveBytecodeHash) + analyzed.bytecodeHash)
           )
 
-      val xs = newBuilder.result()
-      if xs.nonEmpty then stamps(className) = xs
+      if newBuilder.nonEmpty then stamps(className) = newBuilder
       else ()
 
-      builder ++= xs
+      builder |= newBuilder
 
     if alreadySeen.contains(javaClassName) then ()
     else
       stamps.get(javaClassName) match
-        case Some(xs) => builder ++= xs
+        case Some(xs) => builder |= xs
         case _        =>
           alreadySeen += javaClassName
           // Note: internalClassDeps uses Scala-encoded class name for companion objects