With mobile layouts, an app would be kind of pointless. There would be really no difference, and I'm actually puzzled as to why certain social media has apps since they look and function literally the same in a mobile browser.
Anyway, I'm still learning my way around responsive design, and I know xenforo's responsive presets aren't perfect. I'm not really rushing to learn it because I actually don't care for it much, but it's kind of a big deal to have the webpage transform based on screensize due to the varying mobile platforms; rotating your screens and blah blah blah, so it's something I've been exploring for a little over a year now. The old method was to have an entirely separate theme that is set to override the main one when it detected a mobile device.
There's not much I can do regarding the loading speeds other than removing images where they may cause issues, but if there are any glaring issues, visually, or something inhibiting functionality, on mobile, please let me know so I can take a look at it and see if I can resolve it. But make sure to specify on which device (so I can get a general screen size), which browser (and version), and on which theme the issue is happening. Thanks.