I'm seeing reports that Outlook installed on Windows 8 is very slow to send and receive. It's fine when the computer is first booted, but after several hours it slows appreciably.
The cause: antivirus scanning email. Disabling the outbound scan and any Outlook add-ins installed by the antivirus software will solve the problem.
Is it safe to disable email scanning? Sure. There is a reason Microsoft did not include it with their Security Essentials: it's not needed. The only way a virus can get into your computer through Outlook is if you open an attachment or click a link to visit an infected website.
The only thing the add-ins do is provide advance warning that a message might be infected. Is that a value? Possibly, but I'm willing to bet most people would look at the message and say 'junk' anyway.
Scanning outbound messages has even less value. If the anti-virus is doing its job, you won't be infected and won't send infected messages. Not to mention the fact that very few, if any, exploits are going to send mail using your email client. They are more likely to phish for your username and password then use it to send messages from another computer, bypassing your mail client and computer completely.
What about infected attachments? Most people will identify the messages as junk and won't open the attachment. If a person is fooled and opens the attachment, Outlook first writes it to the SecureTemp folder, where the antivirus software scans it. If the attachment is infected, you'll be notified. If an infected document is not identified when it's opened, its not going to be identified when it is scanned by an Outlook add-in during send and receive.
Is it safe to use the reading pane? Sure, as long as you are using Outlook 2000 SP1 or later with all updates installed. If you are using an updated version of Outlook, the reading pane is a tiny bit safer than opening the messages and a whole lot faster.
Be smart. If a message looks suspicious, don't click links or open attachments.
Keep your anti-virus updated. It doesn't matter if mail is scanned as it arrives if the anti-virus definitions are outdated.
Use a strong password and change it if you are suspicious. If you have even the slightest suspicion that your email account was hacked, immediately change your password.
Sign out. When you read mail in a browser, don't just close the browser - sign out.