I recently tested it, and even on a desktop browser "new" Reddit loads slower than old.reddit.com when e.g. navigating from the thread list into an individual thread – the initial page load isn't substantially faster with new Reddit, even though the server has to serve less dynamic content, and then you always spend another second or two waiting for the comments to actually load.

Whereas with old.reddit.com the initial page load might possibly be minimally slower, but end-to-end it's still noticeably faster, because everything, including the comments, loads in one go.

On an underpowered phone with mediocre JavaScript-performance it's even worse, so old.reddit.com is even more preferable (besides the fact that you're also getting around all the "are you sure you don't want to rather view this in the app" nonsense that way), even though it means having to rely on the browser's font inflation-algorithm for getting readable text instead of having a layout that's natively written to accommodate small screen sizes.