diff --git a/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala b/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala
index 0aa3b65437fa..f11a27c43530 100644
--- a/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala
+++ b/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala
@@ -10,6 +10,7 @@ import reporting.trace
 import dotty.tools.dotc.transform.SymUtils._
 import Decorators._
 import Constants.Constant
+import scala.collection.mutable
 
 import scala.annotation.tailrec
 
@@ -698,7 +699,10 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
 
   /** The symbols defined locally in a statement list */
   def localSyms(stats: List[Tree])(using Context): List[Symbol] =
-    for (stat <- stats if stat.isDef && stat.symbol.exists) yield stat.symbol
+    val locals = new mutable.ListBuffer[Symbol]
+    for stat <- stats do
+      if stat.isDef && stat.symbol.exists then locals += stat.symbol
+    locals.toList
 
   /** If `tree` is a DefTree, the symbol defined by it, otherwise NoSymbol */
   def definedSym(tree: Tree)(using Context): Symbol =
diff --git a/compiler/src/dotty/tools/dotc/ast/Trees.scala b/compiler/src/dotty/tools/dotc/ast/Trees.scala
index 922229d1c4b7..c422642879af 100644
--- a/compiler/src/dotty/tools/dotc/ast/Trees.scala
+++ b/compiler/src/dotty/tools/dotc/ast/Trees.scala
@@ -846,7 +846,11 @@ object Trees {
     override def isEmpty: Boolean = trees.isEmpty
     override def toList: List[Tree[T]] = flatten(trees)
     override def toString: String = if (isEmpty) "EmptyTree" else "Thicket(" + trees.mkString(", ") + ")"
-    override def span: Span = trees.foldLeft(NoSpan) ((span, t) => span union t.span)
+    override def span: Span =
+      def combine(s: Span, ts: List[Tree[T]]): Span = ts match
+        case t :: ts1 => combine(s.union(t.span), ts1)
+        case nil => s
+      combine(NoSpan, trees)
 
     override def withSpan(span: Span): this.type =
       mapElems(_.withSpan(span)).asInstanceOf[this.type]
@@ -1375,7 +1379,12 @@ object Trees {
       // Ties the knot of the traversal: call `foldOver(x, tree))` to dive in the `tree` node.
       def apply(x: X, tree: Tree)(using Context): X
 
-      def apply(x: X, trees: Traversable[Tree])(using Context): X = trees.foldLeft(x)(apply)
+      def apply(x: X, trees: List[Tree])(using Context): X = trees match
+        case tree :: rest =>
+          apply(apply(x, tree), rest)
+        case Nil =>
+          x
+
       def foldOver(x: X, tree: Tree)(using Context): X =
         if (tree.source != ctx.source && tree.source.exists)
           foldOver(x, tree)(using ctx.withSource(tree.source))
diff --git a/compiler/src/dotty/tools/dotc/config/Feature.scala b/compiler/src/dotty/tools/dotc/config/Feature.scala
index ed804ff4a7af..12665f88dc2c 100644
--- a/compiler/src/dotty/tools/dotc/config/Feature.scala
+++ b/compiler/src/dotty/tools/dotc/config/Feature.scala
@@ -68,7 +68,9 @@ object Feature:
 
   def sourceVersion(using Context): SourceVersion =
     if ctx.compilationUnit == null then sourceVersionSetting
-    else ctx.compilationUnit.sourceVersion.getOrElse(sourceVersionSetting)
+    else ctx.compilationUnit.sourceVersion match
+      case Some(v) => v
+      case none => sourceVersionSetting
 
   def migrateTo3(using Context): Boolean =
     sourceVersion == `3.0-migration` || enabledBySetting(nme.Scala2Compat)
diff --git a/compiler/src/dotty/tools/dotc/core/Contexts.scala b/compiler/src/dotty/tools/dotc/core/Contexts.scala
index d75a60255003..5ef96561b8d9 100644
--- a/compiler/src/dotty/tools/dotc/core/Contexts.scala
+++ b/compiler/src/dotty/tools/dotc/core/Contexts.scala
@@ -680,11 +680,8 @@ object Contexts {
     final def retractMode(mode: Mode): c.type = c.setMode(c.mode &~ mode)
   }
 
-  /** Test `op` in a fresh context with a typerstate that is not committable.
-   *  The passed context may not survive the operation.
-   */
-   def explore[T](op: Context ?=> T)(using Context): T =
-    util.Stats.record("Context.test")
+  private def exploreCtx(using Context): Context =
+    util.Stats.record("explore")
     val base = ctx.base
     import base._
     val nestedCtx =
@@ -700,13 +697,15 @@ object Contexts {
     exploresInUse += 1
     val nestedTS = nestedCtx.typerState
     nestedTS.init(ctx.typerState, ctx.typerState.constraint)
-    val result =
-      try op(using nestedCtx)
-      finally
-        nestedTS.reporter.asInstanceOf[ExploringReporter].reset()
-        exploresInUse -= 1
-    result
-  end explore
+    nestedCtx
+
+  private def wrapUpExplore(ectx: Context) =
+    ectx.reporter.asInstanceOf[ExploringReporter].reset()
+    ectx.base.exploresInUse -= 1
+
+  inline def explore[T](inline op: Context ?=> T)(using Context): T =
+    val ectx = exploreCtx
+    try op(using ectx) finally wrapUpExplore(ectx)
 
   /** The type comparer of the kind created by `maker` to be used.
    *  This is the currently active type comparer CMP if
@@ -716,6 +715,7 @@ object Contexts {
    *  In other words: tracking or explaining is a sticky property in the same context.
    */
   private def comparer(using Context): TypeComparer =
+    util.Stats.record("comparing")
     val base = ctx.base
     if base.comparersInUse > 0
        && (base.comparers(base.comparersInUse - 1).comparerContext eq ctx)
diff --git a/compiler/src/dotty/tools/dotc/core/Denotations.scala b/compiler/src/dotty/tools/dotc/core/Denotations.scala
index c20be8ee252f..a9eb9f56caaf 100644
--- a/compiler/src/dotty/tools/dotc/core/Denotations.scala
+++ b/compiler/src/dotty/tools/dotc/core/Denotations.scala
@@ -277,7 +277,7 @@ object Denotations {
      *  single-denotations that do not satisfy the predicate are left alone
      *  (whereas suchThat would map them to NoDenotation).
      */
-    def disambiguate(p: Symbol => Boolean)(using Context): SingleDenotation = this match {
+    inline def disambiguate(inline p: Symbol => Boolean)(using Context): SingleDenotation = this match {
       case sdenot: SingleDenotation => sdenot
       case mdenot => suchThat(p) orElse NoQualifyingRef(alternatives)
     }