First I would check if the Trend software is part of the issue by restarting Windows in "Safe mode with networking". If Thunderbird works better then it suggests Trend is the cause of the delays, and I would look into how to exclude Thunderbird from its scanning. If Windows' safe mode doesn't help, I'd try Thunderbird in its own safe mode just to rule about any extensions, add-ons or other customizations,. Generally we advise against allowing real-time scanning by anti-virus software to act on email. Thunderbird stores all the messages you see in a folder in one big file, and if an anti-virus product thinks a message is dangerous, it is likely to quarantine the file that contains the offending message, and that means the whole folder goes with it. The "on-demand" part of your security software should be allowed to continue to operate, so it will step forward if you try to run anything that arrived as an attachment.