It would certainly be faster, though the idea behind the proxy is that it prevents Reddit correlating browsing habits to a specific source (assuming the one hosting the libreddit instance is deemed trustworthy).
https://reddit.com/.json from an incognito window shows me the exact same posts in the same order as libreddit.
I think the argument made is regarding tracking. If I understand this architecture correctly, the use of a proxy would mean reddit can't correlate your browsing habits, as they'll be correlating all browsing habits of people using the proxy.