Microsoft plans next month to roll out the first major update to its Windows Vista operating system, which was introduced in January 2007. There have been a number of smaller patches to Vista, but this one, called Service Pack 1, is pretty large, a 65-megabyte download, and includes hundreds of small fixes and improvements, including some performance gains.

The arrival of a large update like this isn’t a sign of trouble, or even unusual. Microsoft has routinely issued these large “service packs” periodically for Windows. And just this week, its competitor, Apple, unleashed an even larger update for its new operating system, Leopard.

Even though they can take a long time to download and install, such updates are generally a good thing for consumers. Microsoft will automatically deliver SP1, as the company calls it, through its normal updating mechanism, built into Windows. The update is free.

However, based on my tests of Vista SP1, I believe that for most average consumers, it will likely be a nonevent, and for others it will be disappointing. Many of its benefits are aimed at corporations and power users, or are under-the-hood fixes that are hard to discern. For mainstream users, it adds no significant, visible features to Vista, and changes little or nothing about the way the operating system looks and works.

Also, SP1 doesn’t resolve some of the most annoying flaws in Vista, including slow start-ups and reboots, and a security system that nags you too much and requires add-on anti-virus software. I guess these problems will either never be fixed fully or will have to wait for SP2.

While Vista SP1 does deliver some performance improvements in certain scenarios, it can actually temporarily degrade performance — including making reboots even slower — because of a quirk in the update process. This slowdown should go away in a few days, the company says.

On balance, the update is probably worth installing, especially since Microsoft will deliver it automatically. But I wouldn’t rush to grab it and I wouldn’t expect much from it. One note: you can’t install SP1 until you have installed a couple of other patches first. These will also be distributed automatically.

I installed Vista SP1 on two computers that had come with the original Vista preinstalled: a 10-month-old Sony Vaio SZ laptop and a two-month-old Dell XPS One desktop. Because the automatic download distribution isn’t yet in place, Microsoft sent me the update on a disk, which also included the prerequisite patches. In each case, the upgrade took a little over an hour and went smoothly. During the process, the computers rebooted multiple times, but it was all automatic and didn’t require user intervention.

After the installation, the computers functioned normally. I tested three of the performance improvements Microsoft claims for SP1. The first involved speeding up the copying of hefty folders containing large numbers of files. On both machines, copying a folder containing over 700 files totaling almost 700 megabytes took less than half as long with SP1 as it had with the original Vista.

I also tested how long it took both machines to awaken from a hibernation or sleep state and be ready for work. For these tests, I began with each machine running Microsoft Word, Microsoft Outlook and the Firefox Web browser, then I forced them into sleep and hibernation mode.

By my definition, “ready for work” means that Vista’s circular delay indicator has gone away, the software that loads at start-up has finished launching and the computer has fully reconnected to its wired or wireless network. On both of my test machines, SP1 improved the recovery time from sleep or hibernation, shaving one to 10 seconds from the procedures.

Microsoft doesn’t claim SP1 will improve the speed of cold starts and reboots under Vista, but I tested these anyway. To my horror, I found that SP1 actually made rebooting — already slower than on comparable Windows XP computers or Macintoshes — even slower.

Microsoft explained that this was due to the fact that installing SP1 erases certain data used by Vista to speed up program launching. It takes the system a few days to build this data back up, the company says. Until then, it says, overall performance, including reboots, can be slower under SP1 than under original Vista.

Microsoft provided me with a method that would rebuild this program-launching data more quickly, at least for the common programs I was using in my tests. Once I followed that method, rebooting time returned to its former state — still too slow for my taste, but at least not worse.

In briefing me on SP1, Microsoft made a big point of saying that great progress had been made in the past year in making Vista work properly with add-on devices, such as printers. I tried my 2003-vintage Hewlett-Packard printer, which hadn’t worked properly with the original Vista. It still didn’t work well with SP1.

So, Vista SP1 is a step forward, at least after a few days of use. But it’s not a big step.

Email me at mossberg@wsj.com. Find all my columns and videos online, free, at the new All Things Digital Web site, http://walt.allthingsd.com.