I tried on several machines (PC & MAC) at several places with different internet speed, but always got Error 504:
The gateway did not receive a timely response from the upstream server or application.
Apache/2.4.41 (Ubuntu) Server at render.openstreetmap.org Port 443
Asking for an SVG from the OSM website makes an API call (MAP). It is this API call which is expensive. As I said you need to look at an alternative way of getting the data/image, i.e., not via the main OSM website.
That’s not quite accurate. Rendering does not involve any map API call at all. The whole process is based on the well known Mapnik stack to render images directly on a tile server, which obviously uses an osm2pgsql database underneath. What’s consuming lots of resources here is the the large scale rendering.
You’re right of course this is the render call whereas the MAP API call which is what export does is usually deprecated for similar reasons. I think perhaps long ago these were all available under export (xml, svg & png) rather than via share, but it’s a very long time since I’ve even thought of using them because of the high likelihood of it not working.