from what I read the best thing is to reinstall Thunderbird.

Not sure what you read, but it was nonsense. Profile files are stored separately from the Thunderbird program files. The program files are static and do not change (except when you upgrade the Thunderbird application version). Because we store profile and program files separately, you can uninstall Thunderbird without losing your messages and settings, and if something goes wrong with a Thunderbird update your information will still be available. It also means that you don't have to reinstall Thunderbird in order to delete your information or troubleshoot a problem.

I am having trouble with very slow downloading of messages,

Please explain 'very slow downloading' in more detail. What exactly does 'very slow' mean?

I am having trouble with ... duplicate contacts

You can try to find an add-on to sort this out. https://addons.thunderbird.net/thunderbird/

and a number of other irks

I'm not going to guess what these might be.

if i uninstall thunderbird will i lose all my drafts, contacts addons etc. i would reinstall but not if i lose all my settings

what happens if i install over it

Mary,

please start a new topic for your problem. https://support.mozilla.org/questions/new

i think i did but do not think it worked