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+# Benchmarking
+
+## Debug Builds
+
+Debug builds (\--enable-debug) and non-optimized builds
+(\--disable-optimize) are *much* slower. Any performance metrics
+gathered by such builds are largely unrelated to what would be found in
+a release browser.
+
+## Rust optimization level
+
+Local optimized builds are [compiled with rust optimization level 1 by
+default](https://groups.google.com/forum/#!topic/mozilla.dev.platform/pN9O5EB_1q4),
+unlike Nightly builds, which use rust optimization level 2. This setting
+reduces build times significantly but comes with a serious hit to
+runtime performance for any rust code ([for example stylo and
+webrender](https://groups.google.com/d/msg/mozilla.dev.platform/pN9O5EB_1q4/ooXNuqMECAAJ)).
+Add the following to your [mozconfig] in order to build with level 2:
+
+```
+ac_add_options RUSTC_OPT_LEVEL=2
+```
+
+## Profile Guided Optimization (PGO)
+[Profile Guided
+Optimization](/build/buildsystem/pgo.rst#profile-guided-optimization) is
+disabled by default and may improve runtime by up to 20%. However, it takes a
+long time to build. To enable, add the following to your [mozconfig]:
+```
+ac_add_options MOZ_PGO=1
+```
+
+## GC Poisoning
+
+Many Firefox builds have a diagnostic tool that causes crashes to happen
+sooner and produce much more actionable information, but also slow down
+regular usage substantially. In particular, \"GC poisoning\" is used in
+all debug builds, and in optimized Nightly builds (but not opt Developer
+Edition or Beta builds). The poisoning can be disabled by setting the
+environment variable
+
+```
+    JSGC_DISABLE_POISONING=1
+```
+
+before starting the browser.
+
+## Async Stacks
+
+Async stacks no longer impact performance since **Firefox 78**, as
+{{bug(1601179)}} limits async stack capturing to when DevTools is
+opened.
+
+Another option that is on by default in non-release builds is the
+preference javascript.options.asyncstack, which provides better
+debugging information to developers. Set it to false to match a release
+build. (This may be disabled for many situations in the future. See
+{{bug(1280819)}}.
+
+## Accelerated Graphics
+
+Especially on Linux, accelerated graphics can sometimes lead to severe
+performance problems even if things look ok visually. Normally you would
+want to leave acceleration enabled while profiling, but on Linux you may
+wish to disable accelerated graphics (Preferences -\> Advanced -\>
+General -\> Use hardware acceleration when available).
+
+## Flash Plugin
+
+If you are profiling real websites, you should disable the Adobe Flash
+plugin so you are testing Firefox code and not Flash jank problems. In
+about:addons \> Plugins, set Shockwave Flash to \"Never Activate\".
+
+## Timer Precision
+
+Firefox reduces the precision of the Performance APIs and other clock
+and timer APIs accessible to Web Content. They are currently reduce to a
+multiple of 2ms; which is controlled by the privacy.reduceTimerPrecision
+about:config flag.
+
+The exact value of the precision is controlled by the
+privacy.resistFingerprinting.reduceTimerPrecision.microseconds
+about:config flag.
+
+## Profiling tools
+
+Currently the Gecko Profiler has limitations in the UI for inverted call
+stack top function analysis which is very useful for finding heavy
+functions that call into a whole bunch of code. Currently such functions
+may be easy to miss looking at a profile, so feel free to *also* use
+your favorite native profiler. It also lacks features such as
+instruction level profiling which can be helpful in low level profiling,
+or finding the hot loop inside a large function, etc. Some example tools
+include Instruments on OSX (part of XCode), [RotateRight
+Zoom](http://www.rotateright.com/) on Linux (uses perf underneath), and
+Intel VTune on Windows or Linux.
+
+[mozconfig]: /setup/configuring_build_options.rst#using-a-mozconfig-configuration-file