diff --git a/browser/devtools/debugger/debugger-toolbar.js b/browser/devtools/debugger/debugger-toolbar.js
index 2a9ecacdac1b..23c034a89e6e 100644
--- a/browser/devtools/debugger/debugger-toolbar.js
+++ b/browser/devtools/debugger/debugger-toolbar.js
@@ -5,6 +5,10 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 "use strict";
 
+// A time interval sufficient for the options popup panel to finish hiding
+// itself.
+const POPUP_HIDDEN_DELAY = 100; // ms
+
 /**
  * Functions handling the toolbar view: close button, expand/collapse button,
  * pause/resume and stepping buttons etc.
@@ -233,6 +237,13 @@ OptionsView.prototype = {
     this._button.removeAttribute("open");
   },
 
+  /**
+   * Listener handling the 'gear menu' popup hidden event.
+   */
+  _onPopupHidden: function() {
+    window.dispatchEvent(document, "Debugger:OptionsPopupHidden");
+  },
+
   /**
    * Listener handling the 'pause on exceptions' menuitem command.
    */
@@ -269,10 +280,19 @@ OptionsView.prototype = {
    * Listener handling the 'show original source' menuitem command.
    */
   _toggleShowOriginalSource: function() {
+    function reconfigure() {
+      window.removeEventListener("Debugger:OptionsPopupHidden", reconfigure, false);
+      // The popup panel needs more time to hide after triggering onpopuphidden.
+      window.setTimeout(function() {
+        DebuggerController.reconfigureThread(pref);
+      }, POPUP_HIDDEN_DELAY);
+    }
+
     let pref = Prefs.sourceMapsEnabled =
       this._showOriginalSourceItem.getAttribute("checked") == "true";
 
-    DebuggerController.reconfigureThread(pref);
+    // Don't block the UI while reconfiguring the server.
+    window.addEventListener("Debugger:OptionsPopupHidden", reconfigure, false);
   },
 
   _button: null,
diff --git a/browser/devtools/debugger/debugger.xul b/browser/devtools/debugger/debugger.xul
index 3b4b9a1a86ae..cc4d0ce4b90f 100644
--- a/browser/devtools/debugger/debugger.xul
+++ b/browser/devtools/debugger/debugger.xul
@@ -145,7 +145,8 @@
     <menupopup id="debuggerPrefsContextMenu"
                position="before_end"
                onpopupshowing="DebuggerView.Options._onPopupShowing()"
-               onpopuphiding="DebuggerView.Options._onPopupHiding()">
+               onpopuphiding="DebuggerView.Options._onPopupHiding()"
+               onpopuphidden="DebuggerView.Options._onPopupHidden()">
       <menuitem id="pause-on-exceptions"
                 type="checkbox"
                 label="&debuggerUI.pauseExceptions;"
diff --git a/browser/devtools/debugger/test/browser_dbg_source_maps-02.js b/browser/devtools/debugger/test/browser_dbg_source_maps-02.js
index 882bd7e77c96..ff94d36e315f 100644
--- a/browser/devtools/debugger/test/browser_dbg_source_maps-02.js
+++ b/browser/devtools/debugger/test/browser_dbg_source_maps-02.js
@@ -80,6 +80,7 @@ function testToggleGeneratedSource() {
   gDebugger.DebuggerView.Options._showOriginalSourceItem.setAttribute("checked",
                                                                       "false");
   gDebugger.DebuggerView.Options._toggleShowOriginalSource();
+  gDebugger.DebuggerView.Options._onPopupHidden();
 }
 
 function testSetBreakpoint() {
@@ -157,6 +158,7 @@ function testToggleOnPause() {
   gDebugger.DebuggerView.Options._showOriginalSourceItem.setAttribute("checked",
                                                                       "true");
   gDebugger.DebuggerView.Options._toggleShowOriginalSource();
+  gDebugger.DebuggerView.Options._onPopupHidden();
 }
 
 function resumeAndFinish()
diff --git a/toolkit/devtools/server/actors/script.js b/toolkit/devtools/server/actors/script.js
index f483548a06a3..d34dde33bd61 100644
--- a/toolkit/devtools/server/actors/script.js
+++ b/toolkit/devtools/server/actors/script.js
@@ -716,8 +716,30 @@ ThreadActor.prototype = {
    * Get the script and source lists from the debugger.
    */
   _discoverScriptsAndSources: function TA__discoverScriptsAndSources() {
-    return all([this._addScript(s)
-                for (s of this.dbg.findScripts())]);
+    let promises = [];
+    let foundSourceMaps = false;
+    let scripts = this.dbg.findScripts();
+    for (let s of scripts) {
+      if (s.sourceMapURL && !foundSourceMaps) {
+        foundSourceMaps = true;
+        break;
+      }
+    }
+    if (this._options.useSourceMaps && foundSourceMaps) {
+      for (let s of scripts) {
+        promises.push(this._addScript(s));
+      }
+      return all(promises);
+    }
+    // When source maps are not enabled or not used in the page _addScript is
+    // synchronous, since it doesn't need to wait for fetching source maps, so
+    // resolves immediately. This eliminates a huge slowdown in script-heavy
+    // pages like G+ or chrome debugging, where the GC takes a long time to
+    // clean up after Promise.all.
+    for (let s of scripts) {
+      this._addScriptSync(s);
+    }
+    return resolve(null);
   },
 
   onSources: function TA_onSources(aRequest) {
@@ -1248,6 +1270,40 @@ ThreadActor.prototype = {
     return true;
   },
 
+  /**
+   * Add the provided script to the server cache synchronously, without checking
+   * for any declared source maps.
+   *
+   * @param aScript Debugger.Script
+   *        The source script that will be stored.
+   * @returns true, if the script was added; false otherwise.
+   */
+  _addScriptSync: function TA__addScriptSync(aScript) {
+    if (!this._allowSource(aScript.url)) {
+      return false;
+    }
+
+    this.sources.source(aScript.url);
+    // Set any stored breakpoints.
+    let existing = this._breakpointStore[aScript.url];
+    if (existing) {
+      let endLine = aScript.startLine + aScript.lineCount - 1;
+      // Iterate over the lines backwards, so that sliding breakpoints don't
+      // affect the loop.
+      for (let line = existing.length - 1; line >= 0; line--) {
+        let bp = existing[line];
+        // Only consider breakpoints that are not already associated with
+        // scripts, and limit search to the line numbers contained in the new
+        // script.
+        if (bp && !bp.actor.scripts.length &&
+            line >= aScript.startLine && line <= endLine) {
+          this._setBreakpoint(bp);
+        }
+      }
+    }
+    return true;
+  },
+
   /**
    * Add the provided script to the server cache.
    *