I'd say the inverse is more likely. If you're going to fire a single request to a domain only you are using and you're running a full local resolver, it may make a difference.
For a public CDN: your browser already has the file cached. If it doesn't then it has the domain cached. If it doesn't then the dhcp-provided resolver has it. If it doesn't, then at least it already has the TLD nameserver available immediately, and the TLD can serve that response from very hot cache. It's CDNs job to make sure this happens.
When you have a vanity domain like cdn.example.com, the recursive resolver already knows the nameservers for example.com, so this actually reduces the additional DNS lookups.
Oh, and if the requestor has a large DNS cache upstream, it's already done.
Oh, and if the browser used a pre-fetch, that's already done.
Oh, and if you have already invested your branding effort across 100 sites, maybe you don't want to re-do all that?
Oh, and if you need to cut 50ms from your first time page load, have you considered dropping all the trackers and analysis JS loads? Can you deliver your first page without any JS at all? Can you do it without a database lookup?
Those are all things you should do before killing anyone.
I will have a look at your font suggestions
But I agree one needs to zoom out and put things into its proper perspective.
Disclaimer: I run .dev.
I find this a bit hard to believe without more people justifying or backing this work up.
So yeah, don't worry about your TLD. .io is perfectly fine and companies and people internationally use it.
https://gigaom.com/2014/06/30/the-dark-side-of-io-how-the-u-...