Is it possible to do ajax polling in WordPress with admin-ajax.php for example every 300ms? I have tried this and it appears that doing the ajax request with admin-ajax takes longer than the setTimeOut interval. I have a recursive function with setTimeOut for the purposes of polling a session variable set in the php in order to display a progress bar on the front end, so the interval needs to be pretty quick. As an alternative approach I was able to do an ajax request to a stand-alone php file that echoes the session variable. This doesn't seem to be the prescribed way to do ajax in WordPress. It's working on my system, but another tester is getting an error (404, even though the path is correct).
You idea is equivalent to DDOSing your site, and the difference between the methods is only how fast you will bring the site down.
Wordpress AJAX handling is expensive, and should be limited to user initiated actions. Your second method will be more resilient but you are still going to be wasting server resources as initializing PHP by itself has cost, and usually the server is configured on handle on a limited amount of requests in a time, and your "polling" will leave less available "connection slots" for actual page views.