Neither are high performance, and they do different things.
fetch_feed is for grabbing feeds RSS feeds etc, wp_remote_get is for grabbing arbitrary items.
Neither are fast, and the performance difference between each is negligible or irrelevant, but technically fetch_feed is slower than wp_remote_get, not because it’s faster or slower at grabbing over the network, but because it uses SimplePie to parse the feed ( which you’d end up using anyway if you used wp_remote_get on a feed ).
So even using wp_remote_get, you’d get the Feed in the same amount of time, but then you’d have to write the code to process it afterwards, and I’m sure the SimplePie people will be faster as they’ve had more time to optimise and test than you.
So if you’re grabbing an RSS or Atom feed, use fetch_feed
If you’re grabbing html files, images, or other files, use wp_remote_get
Why don’t you use one of the feed aggregation plugins instead of reinventing the wheel?