I have been using outlook 2016 for mac fore a while and i have added and delted many accounts. Lately it have becomne so slow. I tried to delete ans add accounts, still slow. I tried to make a new profile, still slow. Then i came across a fix from reddit by "Mac Rebel"
Try rebuilding your shared software library cache... This can be done by booting into Safe Mode (holding the shift key on startup), or by running the following Terminal command:
sudo update_dyld_shared_cache -force
Update: I also tested Onyx and it really helped. Remember that always have a backup (or backups) before you are running any "fix"-software.
Download Onyx here.