Extremely slow for me as well (with a fast machine) on Arch linux in both firefox and chrome.
Getting about a million "Access to fetch at 'https://example.com/' from origin 'https://map.mta.info' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled."
