As I understand encoding it is more dependent on your region and language choices than what operating system you are using. I am using Unicode but I am on Win 7. If things are going slow it is usually related to your antivirus getting involved where it is not needed. You can restart windows in Safe mode with networking to boot up with minimal things running, no AV, and see what happens. If this helps you can tell your AV software to leave Thunderbird and its files alone and see if things work better.