Slow attachment upload

- What is your operating system? - What anti-virus/Internet security/VPN software are you running, if any? - Are the attachments on the same disc as Thunderbird (both or either the program itself and your Profile?) - Does it still happen when you restart Thunderbird in Troubleshoot Mode (See <Help | Troubleshoot Mode>.) My first suspicion is the AV scanning the message. They are notorious for taking hold of a file and not letting go. McAfee and Norton are the worst offenders, they're not the only ones. Try disabling it, turning off any email scanning features, or creating an exception for Thunderbird.