When I file or delete messages in my inbox it takes a long time to process them unless I turn off Windows Security virus checker. Surely the messages have been virus checked already when they were first downloaded so why does it need to do it again? If I do any more than a small amount of tidying up my mailbox I have to disable downloads and turn off real time virus checking temporarily, otherwise it takes forever. This is obviously not a good thing to do. Is there any way round this?

Exclude your mailbox directory from active scanning in Window Security.

Windows Security > Virus & threat protection > Virus & threat protection settings > Manage settings > Exclusions

In a default installation you would exclude C:\Pmail\MAIL but if you are unsure of the location of your mailbox directory you can find it from with Pegasus Mail by going to Help > About Pegasus Mail then clicking the Info button.

Thanks - that did the trick! But presumably that means incoming mail is not virus checked.

Actually, that is ok provided you have a basic level of security consciousness. Email hazards are either in attachments, or at the destination of an included link. Not clicking links without first knowing their destination is important. As for attachments, when you open an attached file from within the Attachment tab of the reader window, the file is extracted to your system temporary directory by default. That file gets scanned by the virus checker when it is written. In the event that you have specified a different directory as the Pegasus Mail temporary directory you will want to make sure that it is not excluded from active scanning. For added peace of mind, you can first save the attachment as a file and then manually scan it before opening it.