Platform preview releases are separate from the IE9 Beta, which is intended for public consumption.
The previews are released so that developers can test their code and offer feedback on the upcoming software's underlying engine.
Microsoft launched the first platform preview last March at its MIX10 Web developer conference. The public beta is not being updated with the new JavaScript engine just yet since its larger audience requires a higher level of stability, refinement, and testing.
“IE9 is now at the top spot in JavaScript performance,” said Ryan Gavin, senior director of Internet Explorer.
Gavin said in the year that the Chakra JavaScript engine used by IE9 was first demonstrated, its performance has improved 310 percent.
"The significance of this release is really a by-product of our focus on all-around site performance, making the coding patterns that real-world developers are using in the wild for real-world sites faster," Gavin said.
"Our focus has been on hardware acceleration enabling rich performance of things like graphics, audio, video, and text through hardware acceleration using the power of the PC. With this seventh Platform Preview, we're coupling that with incredibly fast JavaScript."