@Mohammad_Chaaban I think the proxy_set_header Authorization ""; directive should actually go in the location / block. Otherwise it will not work and the credentials that are meant for NGINX are passed to openHAB, and get validated for every request the UI makes, which is why it’s slow.
Also, if your intent is to do password protection (basic authentication) of the entire instance you need to do it solely on the NGINX level (with the .htpasswd file) and NOT enable Basic authentication in Settings > API Security in openHAB.