diff --git a/compiler/src/dotty/tools/dotc/util/LinearSet.scala b/compiler/src/dotty/tools/dotc/util/LinearSet.scala
index 13db1af8c613..075985029923 100644
--- a/compiler/src/dotty/tools/dotc/util/LinearSet.scala
+++ b/compiler/src/dotty/tools/dotc/util/LinearSet.scala
@@ -1,7 +1,7 @@
 package dotty.tools.dotc.util
 import collection.immutable
 
-/** A linear set is a set here after a `+` the previous set value cannot be
+/** A linear set is a set where after a `+` the previous set value cannot be
  *  used anymore. The set is implemented as an immutable set for sizes <= 4
  *  and as a HashSet for larger sizes.
  */
diff --git a/tests/pos-with-compiler/benchSets.scala b/tests/pos-with-compiler/benchSets.scala
new file mode 100644
index 000000000000..8619318956f9
--- /dev/null
+++ b/tests/pos-with-compiler/benchSets.scala
@@ -0,0 +1,192 @@
+type Elem = Object
+
+def newElem() = new Object()
+
+val MissFactor = 2
+
+val Runs = 100          // number of runs to warm-up, and then number of runs to test
+val ItersPerRun = 1000
+val elems = Array.fill(1024 * MissFactor)(newElem())
+
+def testJava =
+  val set = java.util.HashMap[Elem, Elem]()
+  var count = 0
+  var iter = 0
+  while iter < ItersPerRun do
+    var i = 0
+    while i < elems.length do
+      val e = elems(i)
+      if i % MissFactor == 0 then
+        set.put(e, e)
+      i += 1
+    while i > 0 do
+      i -= 1
+      val v = set.get(elems(i))
+      if v != null then
+        count += 1
+    iter += 1
+  count
+
+def testJavaId =
+  val set = java.util.IdentityHashMap[Elem, Elem]()
+  var count = 0
+  var iter = 0
+  while iter < ItersPerRun do
+    var i = 0
+    while i < elems.length do
+      val e = elems(i)
+      if i % MissFactor == 0 then
+        set.put(e, e)
+      i += 1
+    while i > 0 do
+      i -= 1
+      val v = set.get(elems(i))
+      if v != null then
+        count += 1
+    iter += 1
+  count
+
+def testScalaMap =
+  val set = scala.collection.mutable.HashMap[Elem, Elem]()
+  var count = 0
+  var iter = 0
+  while iter < ItersPerRun do
+    var i = 0
+    while i < elems.length do
+      val e = elems(i)
+      if i % MissFactor == 0 then
+        set.update(e, e)
+      i += 1
+    while i > 0 do
+      i -= 1
+      set.get(elems(i)) match
+        case Some(_) => count += 1
+        case None =>
+    iter += 1
+  count
+
+def testAnyRefMap =
+  val set = scala.collection.mutable.AnyRefMap[Elem, Elem]()
+  var count = 0
+  var iter = 0
+  while iter < ItersPerRun do
+    var i = 0
+    while i < elems.length do
+      val e = elems(i)
+      if i % MissFactor == 0 then
+        set.update(e, e)
+      i += 1
+    while i > 0 do
+      i -= 1
+      val v = set.getOrNull(elems(i))
+      if v != null then
+        count += 1
+    iter += 1
+  count
+
+def testDottyMap =
+  val set = dotty.tools.dotc.util.HashMap[Elem, Elem](128)
+  var count = 0
+  var iter = 0
+  while iter < ItersPerRun do
+    var i = 0
+    while i < elems.length do
+      val e = elems(i)
+      if i % MissFactor == 0 then
+        set.update(e, e)
+      i += 1
+    while i > 0 do
+      i -= 1
+      val v = set.lookup(elems(i))
+      if v != null then
+        count += 1
+    iter += 1
+  count
+
+def testDottySet =
+  val set = dotty.tools.dotc.util.HashSet[Elem](64)
+  var count = 0
+  var iter = 0
+  while iter < ItersPerRun do
+    var i = 0
+    while i < elems.length do
+      val e = elems(i)
+      if i % MissFactor == 0 then
+        set += e
+      i += 1
+    while i > 0 do
+      i -= 1
+      if set.contains(elems(i)) then
+        count += 1
+    iter += 1
+  count
+
+def testScalaSet =
+  val set = scala.collection.mutable.HashSet[Elem]()
+  var count = 0
+  var iter = 0
+  while iter < ItersPerRun do
+    var i = 0
+    while i < elems.length do
+      val e = elems(i)
+      if i % MissFactor == 0 then
+        set += e
+      i += 1
+    while i > 0 do
+      i -= 1
+      if set.contains(elems(i)) then
+        count += 1
+    iter += 1
+  count
+
+def testLinearSet =
+  var set = dotty.tools.dotc.util.LinearSet.empty[Elem]
+  var count = 0
+  var iter = 0
+  while iter < ItersPerRun do
+    var i = 0
+    while i < elems.length do
+      val e = elems(i)
+      if i % MissFactor == 0 then
+        set += e
+      i += 1
+    while i > 0 do
+      i -= 1
+      if set.contains(elems(i)) then
+        count += 1
+    iter += 1
+  count
+
+val expected = (elems.size / MissFactor) * ItersPerRun
+
+def profile(name: String, op: => Int) =
+  System.gc()
+  for i <- 0 until 100 do assert(op == expected)
+  val start = System.nanoTime()
+  var count = 0
+  for i <- 0 until 100 do count += op
+  //println(count)
+  assert(count == expected * Runs)
+  println(s"$name took ${(System.nanoTime().toDouble - start)/1_000_000} ms")
+
+@main def Test =
+
+  profile("dotty.tools.dotc.LinearSet", testLinearSet)
+  profile("dotty.tools.dotc.HashSet  ", testDottySet)
+  profile("dotty.tools.dotc.HashMap  ", testDottyMap)
+  profile("scala.collection.HashSet  ", testScalaSet)
+  profile("scala.collection.AnyRefMap", testAnyRefMap)
+  profile("scala.collection.HashMap  ", testScalaMap)
+  profile("java.util.IdentityHashMap ", testJavaId)
+  profile("java.util.HashMap         ", testJava)
+
+  profile("java.util.HashMap         ", testJava)
+  profile("java.util.IdentityHashMap ", testJavaId)
+  profile("scala.collection.HashMap  ", testScalaMap)
+  profile("scala.collection.AnyRefMap", testAnyRefMap)
+  profile("scala.collection.HashSet  ", testScalaSet)
+  profile("dotty.tools.dotc.HashMap  ", testDottyMap)
+  profile("dotty.tools.dotc.HashSet  ", testDottySet)
+  profile("dotty.tools.dotc.LinearSet", testLinearSet)
+
+
diff --git a/tests/pos/cm/ConfManagement.scala b/tests/pos/cm/ConfManagement.scala
deleted file mode 100644
index 276918537c9e..000000000000
--- a/tests/pos/cm/ConfManagement.scala
+++ /dev/null
@@ -1,25 +0,0 @@
-package cm
-
-case class Person(name: String)
-case class Paper(title: String, authors: List[Person], body: String)
-
-class Viewers(val persons: Set[Person])
-
-class ConfManagement(papers: List[Paper], realScore: Map[Paper, Int]) {
-
-  private def hasConflict(ps1: Set[Person], ps2: Iterable[Person]) =
-    ps2.exists(ps1 contains _)
-
-  def viewers(implicit vs: Viewers): Set[Person] =
-    vs.persons
-
-  def score(paper: Paper)(implicit vs: Viewers): Int =
-    if (hasConflict(viewers, paper.authors)) -100
-    else realScore(paper)
-
-  def viewRankings(implicit vs: Viewers): List[Paper] =
-    papers.sortBy(score(_))
-
-  def delegateTo[T](query: Viewers => T, p: Person)(implicit vs: Viewers): T =
-    query(new Viewers(viewers + p))
-}
\ No newline at end of file
diff --git a/tests/run/ConfManagement.check b/tests/run/ConfManagement.check
new file mode 100644
index 000000000000..b40e701f2318
--- /dev/null
+++ b/tests/run/ConfManagement.check
@@ -0,0 +1,4 @@
+Peters
+
+Smith
+Smith, Peters
diff --git a/tests/run/ConfManagement.scala b/tests/run/ConfManagement.scala
new file mode 100644
index 000000000000..f73445b5af08
--- /dev/null
+++ b/tests/run/ConfManagement.scala
@@ -0,0 +1,83 @@
+case class Person(name: String)
+case class Paper(title: String, authors: List[Person], body: String)
+
+object ConfManagement:
+  opaque type Viewers = Set[Person]
+  def viewers(using vs: Viewers) = vs
+  type Viewed[T] = Viewers ?=> T
+
+  class Conference(ratings: (Paper, Int)*):
+    private val realScore = ratings.toMap
+
+    def papers: List[Paper] = ratings.map(_._1).toList
+
+    def score(paper: Paper): Viewed[Int] =
+      if viewers.intersect(paper.authors.toSet).nonEmpty then -100
+      else realScore(paper)
+
+    def rankings: Viewed[List[Paper]] =
+      papers.sortBy(score(_)).reverse
+
+    def ask[T](p: Person, query: Viewed[T]) =
+      query(using Set(p))
+
+    def delegateTo[T](p: Person, query: Viewed[T]): Viewed[T] =
+      query(using viewers + p)
+  end Conference
+end ConfManagement
+
+import ConfManagement._
+
+val Smith = Person("Smith")
+val Peters = Person("Peters")
+val Abel = Person("Abel")
+val Black = Person("Black")
+val Ed = Person("Ed")
+
+val conf = Conference(
+  Paper(
+    title = "How to grow beans",
+    authors = List(Smith, Peters),
+    body = "..."
+  ) -> 92,
+  Paper(
+    title = "Organic gardening",
+    authors = List(Abel, Peters),
+    body = "..."
+  ) -> 83,
+  Paper(
+    title = "Composting done right",
+    authors = List(Black, Smith),
+    body = "..."
+  ) -> 99,
+  Paper(
+    title = "The secret life of snails",
+    authors = List(Ed),
+    body = "..."
+  ) -> 77
+)
+
+def highlyRankedProlificAuthors(asking: Person): Set[Person] =
+  def query: Viewed[Set[Person]] =
+    val highlyRanked =
+      conf.rankings.takeWhile(conf.score(_) > 80).toSet
+    for
+      p1 <- highlyRanked
+      p2 <- highlyRanked
+      author <- p1.authors
+      if p1 != p2 && p2.authors.contains(author)
+    yield author
+  conf.ask(asking, query)
+
+def testAs(person: Person) =
+  println(
+    highlyRankedProlificAuthors(asking = person)
+      .map(_.name)
+      .mkString(", "))
+
+@main def Test =
+  testAs(Black)
+  testAs(Smith)
+  testAs(Abel)
+  testAs(Ed)
+