Early users of Outlook 2007, the latest version of Microsoft's market leading email client, are voicing widespread complaints about the software's sluggish performance.

Symptoms being reported by users include temporary freezes when commands are executed or windows are opened, an inability of Outlook to keep up with text as it is being typed and slowness in sending and receiving emails.

Most of the problems don't appear to be the result of underpowered PCs or of faulty or misconfigured email servers. Instead, users say – and Microsoft acknowledges – that the underlying cause is changes made under Outlook's hood to accommodate new features such as RSS feeds and indexing for faster searches.

Some bloggers working with beta versions of Outlook 2007 have been complaining for months. Even loyalists such as Microsoft's Most Valuable Professionals – the company's elite corps of unpaid technical helpers – are grumbling.

"A lot of MVPs are complaining about Outlook's performance," said Paul Robichaux, an MVP who works as a Microsoft Exchange consultant at 3Sharp, a US IT services firm.

For Robichaux, Outlook 2007 tends to freeze when he's trying to download email from his Exchange Server. It also holds outgoing email indefinitely instead of sending the messages promptly, he said. Robichaux is sure that the problem isn't with Exchange because other, near-identical PCs that are running Outlook 2007 and are connected to the same email server work fine. "Clearly, there are some things I don't understand," he said.

Jason Clarke, who works in the technical sales and marketing department at Wenco International Mining Systems in Richmond, British Columbia, receives about 100 emails on a daily basis. He said that on his PC, Outlook 2007 "hangs completely for three to seven seconds typically, and up to 20 seconds in worst cases when new mail is being downloaded."

Turning off Outlook add-ins that he has installed, such as ClearContext's e-mail management software and Caelo Software's Nelson Email Organiser, "does improve matters marginally, but not nearly as much as it should," Clarke said. "The hesitation is still very noticeable, jarring even."

Clarke, who oversees Wenco's email system, has posted information about Outlook's slow performance on his personal blog. He blames the problems on the new email indexing engine that Outlook 2007 shares with Windows Vista.

The indexing allows searches to be done almost instantly in Outlook 2007, a vast improvement over Outlook 2003. However, the process also appears to be CPU-intensive. Microsoft recommends that users put indexing on a regular schedule instead of letting it run constantly in the background.

Peter O'Kelly, an analyst at Burton Group, said he thinks some of the sluggishness arises from Microsoft's decision to let users download RSS feed data into Outlook's local email file in the form of either a .pst or .ost file.

Adding RSS feeds can quickly swell the inboxes of many users to more than 2GB of data, according to O'Kelly. He said that causes Outlook 2007, especially when it's running on PCs that don't have large amounts of memory, to write to the hard drive much more often than it typically does – resulting in performance slowdowns. Hopefully, Microsoft will be able to better tune that part of the software before the next major release, O'Kelly said.

Outlook is rarely in the spotlight when Microsoft talks up its Office suite. But because Outlook has long combined email with calendar, contact management and other functions, for many users it is their most heavily used – and favourite – application. As a result, latency that might be marginally acceptable for other applications is infuriating to many Outlook devotees and power users.

Although the performance issues are more annoyances than major problems and aren't being universally experienced by early adopters of Office 2007, some users say they are fed up and plan to go back to Outlook 2003. Others who have yet to upgrade plan to stick with Outlook 2003 for the time being.

Last month, Microsoft posted a technical help document on its support Web site offering advice on solving the .pst and .ost file problems. For example, the software vendor recommends that users reduce the sizes of their .pst and .ost files by deleting emails, splitting up large files into smaller ones while archiving older messages and restricting the amount of email and attachments that can be downloaded locally.

Third-party guides have also started appearing on the web, such as one posted in a blog on ClearContext's web site.

Asked whether fixes would appear in Outlook 2007's first service-pack release, a Microsoft spokeswoman said that the company "is definitely looking at how to fix this issue," but otherwise declined to comment. The blog of Michael Affronti, Microsoft's Outlook programme manager, has also been silent on the issue.

Teney Takahashi, an analyst at Radicati Group, downplayed the performance complaints. "My impression is that these are typical of any new release," he said.

Takahashi predicts that 18 million users will be running Outlook 2007 by the end of the year and that by 2010, the new release will overtake prior versions of Outlook in popularity. On the other hand, Takahashi said he thinks that many companies will wait until the Service Pack 1 release of Outlook 2007 before deploying the software.