--- a/toolkit/components/telemetry/docs/collection/events.rst
+++ b/toolkit/components/telemetry/docs/collection/events.rst
@@ -129,16 +129,20 @@ Public JS API
 Record a registered event.
 
 * ``value``: Optional, may be ``null``. A string value, limitted to 80 bytes.
 * ``extra``: Optional. An object with string keys & values. Key strings are limitted to what was registered. Value strings are limitted to 80 bytes.
 
 Throws if the combination of ``category``, ``method`` and ``object`` is unknown.
 Recording an expired event will not throw, but print a warning into the browser console.
 
+.. warning::
+
+  Event Telemetry recording is designed to be cheap, not free. If you wish to record events in a performance-sensitive piece of code, store the events locally and record them only after the performance-sensitive piece ("hot path") has completed.
+
 Example:
 
 .. code-block:: js
 
   Services.telemetry.recordEvent("ui", "click", "reload-btn");
   // event: [543345, "ui", "click", "reload-btn"]
   Services.telemetry.recordEvent("ui", "search", "search-bar", "google");
   // event: [89438, "ui", "search", "search-bar", "google"]

--- a/toolkit/components/telemetry/docs/collection/histograms.rst
+++ b/toolkit/components/telemetry/docs/collection/histograms.rst
@@ -255,16 +255,20 @@ Probes in native code can also use the `
   void AccumulateTimeDelta(ID id, TimeStamp start, TimeStamp end = TimeStamp::Now());
 
 The histogram names declared in ``Histograms.json`` are translated into constants in the ``mozilla::Telemetry`` namespace:
 
 .. code-block:: cpp
 
   mozilla::Telemetry::Accumulate(mozilla::Telemetry::STARTUP_CRASH_DETECTED, true);
 
+.. warning::
+
+  Telemetry accumulations are designed to be cheap, not free. If you wish to accumulate values in a performance-sensitive piece of code, store the accumualtions locally and accumulate after the performance-sensitive piece ("hot path") has completed.
+
 The ``Telemetry.h`` header also declares the helper classes ``AutoTimer`` and ``AutoCounter``. Objects of these types automatically record a histogram value when they go out of scope:
 
 .. code-block:: cpp
 
   nsresult
   nsPluginHost::StopPluginInstance(nsNPAPIPluginInstance* aInstance)
   {
     Telemetry::AutoTimer<Telemetry::PLUGIN_SHUTDOWN_MS> timer;

--- a/toolkit/components/telemetry/docs/collection/scalars.rst
+++ b/toolkit/components/telemetry/docs/collection/scalars.rst
@@ -42,16 +42,20 @@ Probes in native code can use the more c
     void ScalarSet(mozilla::Telemetry::ScalarID aId, bool aValue);
     void ScalarSetMaximum(mozilla::Telemetry::ScalarID aId, uint32_t aValue);
 
     void ScalarAdd(mozilla::Telemetry::ScalarID aId, const nsAString& aKey, uint32_t aValue);
     void ScalarSet(mozilla::Telemetry::ScalarID aId, const nsAString& aKey, uint32_t aValue);
     void ScalarSet(mozilla::Telemetry::ScalarID aId, const nsAString& aKey, bool aValue);
     void ScalarSetMaximum(mozilla::Telemetry::ScalarID aId, const nsAString& aKey, uint32_t aValue);
 
+.. warning::
+
+  Scalar operations are designed to be cheap, not free. If you wish to manipulate Scalars in a performance-sensitive piece of code, store the operations locally and change the Scalar only after the performance-sensitive piece ("hot path") has completed.
+
 The YAML definition file
 ========================
 Scalar probes are required to be registered, both for validation and transparency reasons,
 in the `Scalars.yaml <https://dxr.mozilla.org/mozilla-central/source/toolkit/components/telemetry/Scalars.yaml>`_
 definition file.
 
 The probes in the definition file are represented in a fixed-depth, two-level structure: