JS: Improve performance when starting IE using Visual Studio

• When developing web sites in VS, I always start IE by pressing F5 or by right-clicking on the HTML page and clicking on "View in Browser".
• Recently, I was experimenting with HTML5 code to copy the frames of an MP4 video, reverse the frame data and render it back on a canvas.
• The code uses requestAnimationFrame to process the frames and also displays the achieved frames per second (fps).
• This is what I got:

•  Only 4 fps. To analyze the cause, I used xperf as follows:
  • Open a CMD window with admin rights.
  • Set environment variables:
    • set _NT_SYMBOL_PATH=SRV*C:\websymbols*https://msdl.microsoft.com/downloads/symbols
    • set _NT_SYMCACHE_PATH=C:\websymbols
  • Open the web site using IE.
  • Run:
    • xperf -on base
    • ... profiling occurs here, let it run for a few seconds ...
    • xperf -d trace.etl
    • ... trace file gets saved ...
  • Open the trace file in the viewer:
    • xperf trace.etl
  • In the viewer:
    • Click on "Trace --> Load Symbols"
    • Open frame: "CPU Sampling by Process --> iexplorer.exe (Thread ID that shows high CPU usage)"
    • Select graph area with high CPU usage, then right-click on it and click on "Summary Table"
    • Wait for symbols to load and window to appear:

• Sort the table by clicking on the Weight column.
• The screen shot shows that 40.57% of the CPU capacity was spent in the jscript9.dll (Chakra, IE's JS engine) and 15.24% making function calls to "Js::InterpreterStackFrame::ProcessWithDebugging".
• "ProcessWithDebugging" indicates that debugging is taking place, thus adding an overhead to the execution.

• To test the JS code in IE in "release mode", you cannot set the VS build to "Release" as it will make no difference for the client code.
• Instead, keep the IE window open and just pause the JS execution, e.g. by pressing some button that you can add or pausing the video.
• Then open a fresh instance of IE: On Windows 7 or 8, right-click on the IE icon and then click on the Internet Explorer icon on the context menu.
• Copy the URL of the first IE window to the fresh window and see what happens:

• Now the code reaches 54 fps (of 60 fps max.)!
• Taking a look at the trace of this fresh window shows that the JS execution time is now almost insignificant:

• Instead, most of the time is now being spent in the canvas doing matrix operations:

• Comparison of performance difference of the code in my article "JS: Dots are bad, M'kay?":

IE started from VS
array.length: 5881
len: 4678

Math.floor: 6537
| 0: 4566

document not cached: 6452
document cached: 2831

Fresh IE started outside of VS
array.length: 786
len: 664

Math.floor: 869
| 0: 714

document not cached: 3729
document cached: 44

Summary: When doing something that needs JS performance, start a fresh IE and your code may run up to 100 times faster!