I'm sorry but that's definitely not true. It does not prevent things from loading. They still load, but they are scrubbed from the page. A DNS filter like adguard and pihole actually does what you are describing.
Misinformation, again.
uBO prevents DNS requests from being made in the first place for requests which are meant to be blocked. This way the browser will know faster that a network request is meant to be blocked, it does not have to wait for a DNS resolver to respond.
Additionally, as a result of being in-browser, uBO is also able to collapse the placeholder counterpart of any of network requests which were cancelled.
> does not have to wait for a DNS resolver to respond
The browser (or the system) caches DNS responses and the query for a blocked domain won't be repeated for quite some time.