diff --git a/main/src/main/scala/sbt/internal/Act.scala b/main/src/main/scala/sbt/internal/Act.scala
index 982621665..9e595cea3 100644
--- a/main/src/main/scala/sbt/internal/Act.scala
+++ b/main/src/main/scala/sbt/internal/Act.scala
@@ -18,6 +18,7 @@ import java.net.URI
 import sbt.internal.CommandStrings.{ MultiTaskCommand, ShowCommand, PrintCommand }
 import sbt.internal.util.{ AttributeEntry, AttributeKey, AttributeMap, IMap, Settings, Util }
 import sbt.util.Show
+import scala.collection.mutable
 
 final class ParsedKey(val key: ScopedKey[_], val mask: ScopeMask)
 
@@ -73,14 +74,19 @@ object Act {
       defaultConfigs: Option[ResolvedReference] => Seq[String],
       keyMap: Map[String, AttributeKey[_]]
   ): Parser[Seq[Parser[ParsedKey]]] = {
+    val confParserCache: mutable.Map[Option[sbt.ResolvedReference], Parser[ParsedAxis[String]]] =
+      mutable.Map.empty
     def fullKey =
       for {
         rawProject <- optProjectRef(index, current)
         proj = resolveProject(rawProject, current)
-        confAmb <- configIdent(
-          index configs proj,
-          index configIdents proj,
-          index.fromConfigIdent(proj)
+        confAmb <- confParserCache.getOrElseUpdate(
+          proj,
+          configIdent(
+            index.configs(proj),
+            index.configIdents(proj),
+            index.fromConfigIdent(proj)
+          )
         )
         partialMask = ScopeMask(rawProject.isExplicit, confAmb.isExplicit, false, false)
       } yield taskKeyExtra(index, defaultConfigs, keyMap, proj, confAmb, partialMask)
diff --git a/main/src/main/scala/sbt/internal/KeyIndex.scala b/main/src/main/scala/sbt/internal/KeyIndex.scala
index 343f1d20f..5ce2469ac 100644
--- a/main/src/main/scala/sbt/internal/KeyIndex.scala
+++ b/main/src/main/scala/sbt/internal/KeyIndex.scala
@@ -194,6 +194,7 @@ private[sbt] final class ConfigIndex(
   }
 
   def configs: Set[String] = data.keySet
+  private[sbt] lazy val idents: Set[String] = configIdentToName.keySet
 
   // guess Configuration name from an identifier.
   // There's a guessing involved because we could have scoped key that Project is not aware of.
@@ -201,6 +202,7 @@ private[sbt] final class ConfigIndex(
     configIdentToName.getOrElse(ident, Scope.unguessConfigIdent(ident))
 }
 private[sbt] object ConfigIndex
+
 private[sbt] final class ProjectIndex(val data: Map[Option[String], ConfigIndex]) {
   def add(
       id: Option[String],
@@ -234,7 +236,7 @@ private[sbt] final class KeyIndex0(val data: BuildIndex) extends ExtendableKeyIn
   def configs(project: Option[ResolvedReference]): Set[String] = confIndex(project).configs
 
   private[sbt] def configIdents(project: Option[ResolvedReference]): Set[String] =
-    confIndex(project).configs
+    confIndex(project).idents
 
   private[sbt] def fromConfigIdent(proj: Option[ResolvedReference])(configIdent: String): String =
     confIndex(proj).fromConfigIdent(configIdent)
diff --git a/main/src/test/scala/testpkg/CompletionSpec.scala b/main/src/test/scala/testpkg/CompletionSpec.scala
index ff57d02ae..686001859 100644
--- a/main/src/test/scala/testpkg/CompletionSpec.scala
+++ b/main/src/test/scala/testpkg/CompletionSpec.scala
@@ -28,13 +28,13 @@ object CompletionSpec extends Properties {
       property("can complete any project", TestBuild.nonEmptyId.forAll.map { id =>
         complete(projectID = id, line = id.head.toString, expected = id)
       }),
-      // property(
-      //   "can complete any configuration",
-      //   TestBuild.nonEmptyId.forAll.map { name =>
-      //     val cap = name.capitalize
-      //     complete(configName = name, line = cap.head.toString, expected = cap)
-      //   }
-      // ),
+      property(
+        "can complete any configuration",
+        TestBuild.nonEmptyId.forAll.map { name =>
+          val cap = name.capitalize
+          complete(configName = name, line = cap.head.toString, expected = cap)
+        }
+      ),
       // property("can complete any attribute", TestBuild.kebabIdGen.forAll.map { name =>
       //   complete(attributeName = name, line = name.head.toString, expected = name)
       // })