--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -381,17 +381,18 @@ nsDocShell::nsDocShell(BrowsingContext* 
       mSavingOldViewer(false),
       mDynamicallyCreated(false),
       mAffectPrivateSessionLifetime(true),
       mInvisible(false),
       mHasLoadedNonBlankURI(false),
       mBlankTiming(false),
       mTitleValidForCurrentURI(false),
       mIsFrame(false),
-      mSkipBrowsingContextDetachOnDestroy(false) {
+      mSkipBrowsingContextDetachOnDestroy(false),
+      mWatchedByDevtools(false) {
   mHistoryID.m0 = 0;
   mHistoryID.m1 = 0;
   mHistoryID.m2 = 0;
   AssertOriginAttributesMatchPrivateBrowsing();
 
   nsContentUtils::GenerateUUIDInPlace(mHistoryID);
 
   if (gDocShellCount++ == 0) {
@@ -13543,8 +13544,21 @@ bool nsDocShell::GetIsAttemptingToNaviga
       // This is a javascript: load that might lead to a new document,
       // hence a navigation.
       return true;
     }
   }
 
   return false;
 }
+
+NS_IMETHODIMP
+nsDocShell::GetWatchedByDevtools(bool* aWatched) {
+  NS_ENSURE_ARG(aWatched);
+  *aWatched = mWatchedByDevtools;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsDocShell::SetWatchedByDevtools(bool aWatched) {
+  mWatchedByDevtools = aWatched;
+  return NS_OK;
+}

--- a/docshell/base/nsDocShell.h
+++ b/docshell/base/nsDocShell.h
@@ -1240,11 +1240,14 @@ class nsDocShell final : public nsDocLoa
   bool mTitleValidForCurrentURI : 1;
 
   bool mIsFrame : 1;
 
   // If mSkipBrowsingContextDetachOnDestroy is set to true, then when the
   // docshell is destroyed, the browsing context will not be detached. This is
   // for cases where we want to preserve the BC for future use.
   bool mSkipBrowsingContextDetachOnDestroy : 1;
+
+  // Set when activity in this docshell is being watched by the developer tools.
+  bool mWatchedByDevtools : 1;
 };
 
 #endif /* nsDocShell_h__ */

--- a/docshell/base/nsIDocShell.idl
+++ b/docshell/base/nsIDocShell.idl
@@ -1162,9 +1162,14 @@ interface nsIDocShell : nsIDocShellTreeI
    */
   Promise getContentBlockingLog();
 
   /**
    * Return whether this docshell is "attempting to navigate" in the
    * sense that's relevant to document.open.
    */
   [notxpcom, nostdcall] readonly attribute boolean isAttemptingToNavigate;
+
+  /**
+   * Whether developer tools are watching activity in this docshell.
+   */
+  [infallible] attribute boolean watchedByDevtools;
 };

--- a/dom/base/SerializedStackHolder.h
+++ b/dom/base/SerializedStackHolder.h
@@ -44,20 +44,19 @@ class SerializedStackHolder {
   // This returns null on failure, and does not leave an exception on aCx.
   JSObject* ReadStack(JSContext* aCx);
 };
 
 // Construct a stack for the current thread, which may be consumed by the net
 // monitor later on. This may be called on either the main or a worker thread.
 //
 // This always creates a stack, even if the net monitor isn't active for the
-// associated window. Ideally we would only create the stack if the net monitor
-// was active, but there doesn't seem to be an easy way to determine this.
-// The operations this is used with should be rare enough and/or have enough
-// other associated costs that the perf impact is low. See bug 1546736.
+// associated window. The net monitor will only be active if the associated
+// docshell or worker's WatchedByDevtools flag is set, so this should be checked
+// before creating the stack.
 UniquePtr<SerializedStackHolder> GetCurrentStackForNetMonitor(JSContext* aCx);
 
 // If aStackHolder is non-null, this notifies the net monitor that aStackHolder
 // is the stack from which aChannel originates. This must be called on the main
 // thread. This call is synchronous, and aChannel and aStackHolder will not be
 // used afterward. aChannel is an nsISupports object because this can be used
 // with either nsIChannel or nsIWebSocketChannel.
 void NotifyNetworkMonitorAlternateStack(nsISupports* aChannel,

--- a/dom/workers/WorkerLoadInfo.cpp
+++ b/dom/workers/WorkerLoadInfo.cpp
@@ -82,16 +82,17 @@ WorkerLoadInfoData::WorkerLoadInfoData()
     : mLoadFlags(nsIRequest::LOAD_NORMAL),
       mWindowID(UINT64_MAX),
       mReferrerPolicy(net::RP_Unset),
       mFromWindow(false),
       mEvalAllowed(false),
       mReportCSPViolations(false),
       mXHRParamsAllowed(false),
       mPrincipalIsSystem(false),
+      mWatchedByDevtools(false),
       mStorageAccess(nsContentUtils::StorageAccess::eDeny),
       mFirstPartyStorageAccessGranted(false),
       mServiceWorkersTestingInWindow(false),
       mSecureContext(eNotSet) {}
 
 nsresult WorkerLoadInfo::SetPrincipalsOnMainThread(
     nsIPrincipal* aPrincipal, nsIPrincipal* aStoragePrincipal,
     nsILoadGroup* aLoadGroup) {

--- a/dom/workers/WorkerLoadInfo.h
+++ b/dom/workers/WorkerLoadInfo.h
@@ -113,16 +113,17 @@ struct WorkerLoadInfoData {
   uint64_t mWindowID;
 
   net::ReferrerPolicy mReferrerPolicy;
   bool mFromWindow;
   bool mEvalAllowed;
   bool mReportCSPViolations;
   bool mXHRParamsAllowed;
   bool mPrincipalIsSystem;
+  bool mWatchedByDevtools;
   nsContentUtils::StorageAccess mStorageAccess;
   bool mFirstPartyStorageAccessGranted;
   bool mServiceWorkersTestingInWindow;
   OriginAttributes mOriginAttributes;
 
   enum {
     eNotSet,
     eInsecureContext,

--- a/dom/workers/WorkerPrivate.cpp
+++ b/dom/workers/WorkerPrivate.cpp
@@ -2361,16 +2361,17 @@ nsresult WorkerPrivate::GetLoadInfo(JSCo
     loadInfo.mDomain = aParent->Domain();
     loadInfo.mFromWindow = aParent->IsFromWindow();
     loadInfo.mWindowID = aParent->WindowID();
     loadInfo.mStorageAccess = aParent->StorageAccess();
     loadInfo.mOriginAttributes = aParent->GetOriginAttributes();
     loadInfo.mServiceWorkersTestingInWindow =
         aParent->ServiceWorkersTestingInWindow();
     loadInfo.mParentController = aParent->GetController();
+    loadInfo.mWatchedByDevtools = aParent->IsWatchedByDevtools();
   } else {
     AssertIsOnMainThread();
 
     // Make sure that the IndexedDatabaseManager is set up
     Unused << NS_WARN_IF(!IndexedDatabaseManager::GetOrCreate());
 
     nsIScriptSecurityManager* ssm = nsContentUtils::GetSecurityManager();
     MOZ_ASSERT(ssm);
@@ -2481,16 +2482,21 @@ nsresult WorkerPrivate::GetLoadInfo(JSCo
 
       uint32_t perm;
       rv = permMgr->TestPermissionFromPrincipal(
           loadInfo.mLoadingPrincipal, NS_LITERAL_CSTRING("systemXHR"), &perm);
       NS_ENSURE_SUCCESS(rv, rv);
 
       loadInfo.mXHRParamsAllowed = perm == nsIPermissionManager::ALLOW_ACTION;
 
+      nsIDocShell* docShell = globalWindow->GetDocShell();
+      if (docShell) {
+        loadInfo.mWatchedByDevtools = docShell->GetWatchedByDevtools();
+      }
+
       loadInfo.mFromWindow = true;
       loadInfo.mWindowID = globalWindow->WindowID();
       loadInfo.mStorageAccess =
           nsContentUtils::StorageAllowedForWindow(globalWindow);
       loadInfo.mCookieSettings = document->CookieSettings();
       loadInfo.mOriginAttributes =
           nsContentUtils::GetOriginAttributes(document);
       loadInfo.mParentController = globalWindow->GetController();

--- a/dom/workers/WorkerPrivate.h
+++ b/dom/workers/WorkerPrivate.h
@@ -763,16 +763,20 @@ class WorkerPrivate : public RelativeTim
     return mLoadInfo.mOriginAttributes;
   }
 
   // Determine if the SW testing per-window flag is set by devtools
   bool ServiceWorkersTestingInWindow() const {
     return mLoadInfo.mServiceWorkersTestingInWindow;
   }
 
+  bool IsWatchedByDevtools() const {
+    return mLoadInfo.mWatchedByDevtools;
+  }
+
   // Determine if the worker is currently loading its top level script.
   bool IsLoadingWorkerScript() const { return mLoadingWorkerScript; }
 
   // Called by ScriptLoader to track when this worker is loading its
   // top level script.
   void SetLoadingWorkerScript(bool aLoadingWorkerScript) {
     // any thread
     mLoadingWorkerScript = aLoadingWorkerScript;