I've used SimplePie to parse over 20,000 feeds an hour on very modest resources, but there are memory leak issues (in PHP, not SimplePie) that aren't easily resolved in all PHP installations (even with the recommended fix).

You will probably end up having to spawn separate PHP processes to get the throughput if you ever need to parse that amount of feeds (or higher - easy to do once you've solved this problem of course). An upcoming release of PHP is supposed to fix this problem but we'll see.

I use this (as a Drupal module) for http://esciencenews.com

Don't reinvent the wheel, you don't want to custom code this yourself.

I've used Drupal's aggregator module in the past, and it often has issue with the publish dates and/or whether a link already exists.