diff --git a/clang-tools-extra/clangd/index/Background.h b/clang-tools-extra/clangd/index/Background.h
--- a/clang-tools-extra/clangd/index/Background.h
+++ b/clang-tools-extra/clangd/index/Background.h
@@ -72,7 +72,7 @@
     explicit Task(std::function<void()> Run) : Run(std::move(Run)) {}
 
     std::function<void()> Run;
-    llvm::ThreadPriority ThreadPri = llvm::ThreadPriority::Background;
+    llvm::ThreadPriority ThreadPri = llvm::ThreadPriority::Low;
     unsigned QueuePri = 0; // Higher-priority tasks will run first.
     std::string Tag;       // Allows priority to be boosted later.
     uint64_t Key = 0;      // If the key matches a previous task, drop this one.
diff --git a/clang/tools/libclang/CIndex.cpp b/clang/tools/libclang/CIndex.cpp
--- a/clang/tools/libclang/CIndex.cpp
+++ b/clang/tools/libclang/CIndex.cpp
@@ -9025,7 +9025,7 @@
     return;
 
 #if LLVM_ENABLE_THREADS
-  llvm::set_thread_priority(llvm::ThreadPriority::Background);
+  llvm::set_thread_priority(llvm::ThreadPriority::Low);
 #endif
 }
 
diff --git a/llvm/include/llvm/Support/Threading.h b/llvm/include/llvm/Support/Threading.h
--- a/llvm/include/llvm/Support/Threading.h
+++ b/llvm/include/llvm/Support/Threading.h
@@ -233,15 +233,20 @@
   unsigned get_cpus();
 
   enum class ThreadPriority {
+    /// Try to lower current thread's priority as much as possible. Can be used
+    /// for long-running tasks that are not time critical; more energy-efficient
+    /// than Low.
     Background = 0,
-    Default = 1,
+
+    /// Try to lower current thread's priority such that it does not affect
+    /// foreground tasks significantly. This is a good default for long-running,
+    /// latency-insensitive tasks to make sure cpu is not hogged by this task.
+    Low = 1,
+
+    /// Try to restore current thread's priority to default scheduling
+    /// priority.
+    Default = 2,
   };
-  /// If priority is Background tries to lower current threads priority such
-  /// that it does not affect foreground tasks significantly. Can be used for
-  /// long-running, latency-insensitive tasks to make sure cpu is not hogged by
-  /// this task.
-  /// If the priority is default tries to restore current threads priority to
-  /// default scheduling priority.
   enum class SetThreadPriorityResult { FAILURE, SUCCESS };
   SetThreadPriorityResult set_thread_priority(ThreadPriority Priority);
 }
diff --git a/llvm/lib/Support/Unix/Threading.inc b/llvm/lib/Support/Unix/Threading.inc
--- a/llvm/lib/Support/Unix/Threading.inc
+++ b/llvm/lib/Support/Unix/Threading.inc
@@ -18,6 +18,7 @@
 #if defined(__APPLE__)
 #include <mach/mach_init.h>
 #include <mach/mach_port.h>
+#include <pthread/qos.h>
 #endif
 
 #include <pthread.h>
@@ -258,27 +259,28 @@
   // SCHED_OTHER   the standard round-robin time-sharing policy;
   return !pthread_setschedparam(
              pthread_self(),
-             Priority == ThreadPriority::Background ? SCHED_IDLE : SCHED_OTHER,
+             Priority == ThreadPriority::Default ? SCHED_OTHER : SCHED_IDLE,
              &priority)
              ? SetThreadPriorityResult::SUCCESS
              : SetThreadPriorityResult::FAILURE;
 #elif defined(__APPLE__)
-  // https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/getpriority.2.html
-  // When setting a thread into background state the scheduling priority is set
-  // to lowest value, disk and network IO are throttled. Network IO will be
-  // throttled for any sockets the thread opens after going into background
-  // state. Any previously opened sockets are not affected.
-
-  // https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/getiopolicy_np.3.html
-  // I/Os with THROTTLE policy are called THROTTLE I/Os. If a THROTTLE I/O
-  // request occurs within a small time window (usually a fraction of a second)
-  // of another NORMAL I/O request, the thread that issues the THROTTLE I/O is
-  // forced to sleep for a certain interval. This slows down the thread that
-  // issues the THROTTLE I/O so that NORMAL I/Os can utilize most of the disk
-  // I/O bandwidth.
-  return !setpriority(PRIO_DARWIN_THREAD, 0,
-                      Priority == ThreadPriority::Background ? PRIO_DARWIN_BG
-                                                             : 0)
+  // https://developer.apple.com/documentation/apple-silicon/tuning-your-code-s-performance-for-apple-silicon
+  //
+  // Background - Applies to work that isn’t visible to the user and may take significant
+  // time to complete. Examples include indexing, backing up, or synchronizing data. This
+  // class emphasizes energy efficiency.
+  //
+  // Utility - Applies to work that takes anywhere from a few seconds to a few minutes to
+  // complete. Examples include downloading a document or importing data. This class
+  // offers a balance between responsiveness, performance, and energy efficiency.
+  const auto qosClass = [&](){
+    switch (Priority) {
+      case ThreadPriority::Background: return QOS_CLASS_BACKGROUND;
+      case ThreadPriority::Low: return QOS_CLASS_UTILITY;
+      case ThreadPriority::Default: return QOS_CLASS_DEFAULT;
+    }
+  }();
+  return !pthread_set_qos_class_self_np(qosClass, 0)
              ? SetThreadPriorityResult::SUCCESS
              : SetThreadPriorityResult::FAILURE;
 #endif
diff --git a/llvm/lib/Support/Windows/Threading.inc b/llvm/lib/Support/Windows/Threading.inc
--- a/llvm/lib/Support/Windows/Threading.inc
+++ b/llvm/lib/Support/Windows/Threading.inc
@@ -121,7 +121,7 @@
   // priorities of the thread as they were before the thread entered background
   // processing mode.
   return SetThreadPriority(GetCurrentThread(),
-                           Priority == ThreadPriority::Background
+                           Priority != ThreadPriority::Default
                                ? THREAD_MODE_BACKGROUND_BEGIN
                                : THREAD_MODE_BACKGROUND_END)
              ? SetThreadPriorityResult::SUCCESS