Thunderbird constantly lags for me when clicking emails, sometimes locking up for 10-15 seconds. It also has weird issues with how it displays that you have new messages. It will quite often show me I have unread messages that I've actually already read and I need to expand the account and click through every folder to get it to refresh and recognize that there really aren't any unread. I also have an email account on there that its said I have an unread message for the last 3 months, but when I click the arrow and expand the account, the unread bold (1) goes away, then comes back when I collapse the account. Its just permanently incorrectly stuck as unread for some reason.