Are there any settings I need to check/change? Is it possible that SpamSieve can slow the speed of getting/receiving emails?
SpamSieve does not affect sending mail. It’s normal for receiving messages to be slightly slower. This is because SpamSieve asks Mail to download the full message content earlier than it otherwise might so that it can look at the entire message to determine whether it’s spam. Without SpamSieve, Mail might delay downloading some parts of the message until you actually read it. Secondly, it does take some time for SpamSieve itself to process the messages, although this is normally not noticeable.
If receiving mail seems unusually slow, you can measure what SpamSieve and Mail are doing. Open the Activity Monitor program and select Mail in the list. While it’s receiving messages, choose Sample Process from the View menu. Also sample SpamSieve. Then save the two windows to files and send them to me.