diff --git a/toolkit/components/aboutmemory/content/aboutMemory.js b/toolkit/components/aboutmemory/content/aboutMemory.js
index ffe4d0a31978..aa87e9d3672f 100644
--- a/toolkit/components/aboutmemory/content/aboutMemory.js
+++ b/toolkit/components/aboutmemory/content/aboutMemory.js
@@ -720,12 +720,23 @@ function getTreesByProcess(aProcessMemoryReports, aTreesByProcess,
   // "ghost-windows" multi-reporters all the time.  (Note that reports from
   // these multi-reporters can reach here as single reports if they were in the
   // child process.)
+  //
+  // Also ignore the resident-fast reporter; we use the vanilla resident
+  // reporter because it's more important that we get accurate results than
+  // that we avoid the (small) possibility of a long pause when loading
+  // about:memory.
+  //
+  // We don't show both resident and resident-fast because running the resident
+  // reporter can purge pages on MacOS, which affects the results of the
+  // resident-fast reporter.  We don't want about:memory's results to be
+  // affected by the order of memory reporter execution.
 
   function ignoreSingle(aUnsafePath)
   {
     return (isSmapsPath(aUnsafePath) && !gVerbose && !aForceShowSmaps) ||
            aUnsafePath.startsWith("compartments/") ||
-           aUnsafePath.startsWith("ghost-windows/");
+           aUnsafePath.startsWith("ghost-windows/") ||
+           aUnsafePath == "resident-fast";
   }
 
   function ignoreMulti(aMRName)
diff --git a/toolkit/components/telemetry/TelemetryPing.js b/toolkit/components/telemetry/TelemetryPing.js
index a682ad8378b9..f99a39a048ed 100644
--- a/toolkit/components/telemetry/TelemetryPing.js
+++ b/toolkit/components/telemetry/TelemetryPing.js
@@ -56,7 +56,7 @@ const MEM_HISTOGRAMS = {
   "js-compartments/system": "MEMORY_JS_COMPARTMENTS_SYSTEM",
   "js-compartments/user": "MEMORY_JS_COMPARTMENTS_USER",
   "explicit": "MEMORY_EXPLICIT",
-  "resident": "MEMORY_RESIDENT",
+  "resident-fast": "MEMORY_RESIDENT",
   "vsize": "MEMORY_VSIZE",
   "storage-sqlite": "MEMORY_STORAGE_SQLITE",
   "images-content-used-uncompressed":
diff --git a/xpcom/base/nsMemoryReporterManager.cpp b/xpcom/base/nsMemoryReporterManager.cpp
index 8f5907f00dc2..c43016913beb 100644
--- a/xpcom/base/nsMemoryReporterManager.cpp
+++ b/xpcom/base/nsMemoryReporterManager.cpp
@@ -242,6 +242,11 @@ static nsresult GetResident(int64_t *n)
     return NS_OK;
 }
 
+static nsresult GetResidentFast(int64_t *n)
+{
+    return GetResident(n);
+}
+
 #elif defined(XP_MACOSX)
 
 #include <mach/mach_init.h>
@@ -269,16 +274,17 @@ static nsresult GetVsize(int64_t *n)
     return NS_OK;
 }
 
-static nsresult GetResident(int64_t *n)
+// If we're using jemalloc on Mac, we need to instruct jemalloc to purge the
+// pages it has madvise(MADV_FREE)'d before we read our RSS in order to get
+// an accurate result.  The OS will take away MADV_FREE'd pages when there's
+// memory pressure, so ideally, they shouldn't count against our RSS.
+//
+// Purging these pages can take a long time for some users (see bug 789975),
+// so we provide the option to get the RSS without purging first.
+static nsresult GetResident(int64_t *n, bool aDoPurge)
 {
 #ifdef HAVE_JEMALLOC_STATS
-    // If we're using jemalloc on Mac, we need to instruct jemalloc to purge
-    // the pages it has madvise(MADV_FREE)'d before we read our RSS.  The OS
-    // will take away MADV_FREE'd pages when there's memory pressure, so they
-    // shouldn't count against our RSS.
-    //
-    // Purging these pages shouldn't take more than 10ms or so, but we want to
-    // keep an eye on it since GetResident() is called on each Telemetry ping.
+    if (aDoPurge)
     {
       Telemetry::AutoTimer<Telemetry::MEMORY_FREE_PURGED_PAGES_MS> timer;
       jemalloc_purge_freed_pages();
@@ -293,6 +299,16 @@ static nsresult GetResident(int64_t *n)
     return NS_OK;
 }
 
+static nsresult GetResidentFast(int64_t *n)
+{
+    return GetResident(n, /* doPurge = */ false);
+}
+
+static nsresult GetResident(int64_t *n)
+{
+    return GetResident(n, /* doPurge = */ true);
+}
+
 #elif defined(XP_WIN)
 
 #include <windows.h>
@@ -325,6 +341,11 @@ static nsresult GetResident(int64_t *n)
     return NS_OK;
 }
 
+static nsresult GetResidentFast(int64_t *n)
+{
+    return GetResident(n);
+}
+
 #define HAVE_PRIVATE_REPORTER
 static nsresult GetPrivate(int64_t *n)
 {
@@ -377,6 +398,18 @@ NS_FALLIBLE_MEMORY_REPORTER_IMPLEMENT(Resident,
     "but it depends both on other processes being run and details of the OS "
     "kernel and so is best used for comparing the memory usage of a single "
     "process at different points in time.")
+
+NS_FALLIBLE_MEMORY_REPORTER_IMPLEMENT(ResidentFast,
+    "resident-fast",
+    KIND_OTHER,
+    UNITS_BYTES,
+    GetResidentFast,
+    "This reporter measures the same value as the resident memory reporter, but "
+    "it tries to be as fast as possible, at the expense of accuracy.  On most "
+    "platforms this is identical to the vanilla resident reporter, but on MacOS"
+    "in particular, this reporter may over-count our RSS.  You should use "
+    "resident-fast where you care about latency of collection (e.g. in "
+    "telemetry).  Otherwise you should use the regular resident reporter.")
 #endif  // HAVE_VSIZE_AND_RESIDENT_REPORTERS
 
 #ifdef HAVE_PAGE_FAULT_REPORTERS
@@ -665,6 +698,7 @@ nsMemoryReporterManager::Init()
 #ifdef HAVE_VSIZE_AND_RESIDENT_REPORTERS
     REGISTER(Vsize);
     REGISTER(Resident);
+    REGISTER(ResidentFast);
 #endif
 
 #ifdef HAVE_PAGE_FAULT_REPORTERS