It's taking considerably longer than some members of the Windows Insider program expected, but Microsoft appears to be on the verge of releasing an updated Windows 10 preview build. And this time, the company says, it's serious about picking up the pace.

For those already in the preview program, the Fast ring is about to accelerate. On Twitter today, Microsoft's Gabe Aul announced:

No new build today, but we will be posting an announcement in Insider Hub. suggesting people who want lower risk to move to Slow. -- Gabriel Aul (@GabeAul) March 12, 2015

And a new standalone announcement in the Windows Insider Hub app (available only on systems running Windows 10 already) reinforces the message:

We've heard your feedback asking for more frequent builds--and as Gabe mentioned in his recent blog post, we've probably been too conservative about pushing builds to the Fast ring for Windows Insiders. So we're preparing to speed up the build releases for those who want to live life in the Fast lane. The good news is that Insiders who choose Fast will be getting fresher code, with all of the features and fixes, more often. The potential downside is that as we go faster, the builds will likely include more bugs with fewer workarounds. If this doesn't sound like something you want to deal with, now is your time to switch to Slow.

Today's updates are a follow-up from Aul's blog post earlier this week, which confirmed that a new build is in the final stages of testing:

I have a build in hand that we produced on Friday. It was validated by our test automation, and will go out through our internal rings and get installed and used by thousands of people at Microsoft. It is the freshest code with all newest features and fixes. If it passes all of our evaluation criteria it could be in your hands late this week or early next week.

(If you're interested in the minutiae of Windows development and how processes have changed from earlier eras, the entire post is worth reading.)

If the new build appears in update channels tomorrow (Friday, March 13), it will arrive exactly seven weeks after build 9926, which was released January 23 following a holiday break of more than two months.

That is a much more conservative release pace than most Microsoft watchers expected. In a post announcing that January release, Aul called that gap "the longest you should expect to see with the program... We'll get new builds out faster moving forward."

This week's detailed explanation of how the preview build release cycle works included a frank acknowledgment that the Windows 10 schedule hasn't lived up to that promise so far this year. "[W]e've probably been too conservative about pushing builds to the Fast ring for Windows Insiders," Aul admitted

The marquee feature in the next Windows 10 build is the long-anticipated Spartan browser, which was demoed briefly at January's big unveiling but has yet to make its first appearance in a publicly released preview build. Spartan uses a heavily forked version of Microsoft's Trident rendering engine, known as EdgeHTML. It's intended to be the new default browser for Windows 10, with Internet Explorer relegated to legacy duty.

This update should also show off some changes to the Start menu and Cortana. The Start menu, which was completely rewritten in XAML for the last preview release, is due to add transparency and more personalization options. Cortana will be integrated into the Spartan browser, but details of other changes are scant at this point.

There's no word on whether the next build will incorporate the new Mail/Calendar/People app, (which Microsoft demonstrated back in January), as well as promised changes to the Music app. The biggest change in Music is the addition of digital locker capabilities to allow access to personal music libraries via OneDrive.

The new update will be delivered automatically, via Windows Update, on PCs running the Windows 10 preview and configured for the "Fast" ring. The update will be delayed on PCs set to use the "Slow" ring.

For details on how you can evaluate the Windows 10 Preview (and why you might or might not want to), see my FAQ: Windows 10: You've got questions, I've got answers.

According to Aul, Microsoft "could feasibly get multiple builds out in March rather than just one." But that faster pace comes with bigger risks: "The reality is that faster builds to you will include more bugs, and so far we've erred on the side of stability."

For anyone who decides to stay in the Fast ring, the message is clear: Get ready for a bumpier ride.