For reddit you can use Infinity or other reddit clients, I use Infinity and I don't even need an account to subscribe to subredits that I want to follow and for me it seems to be a lot faster then the official reddit app.
If you're using reddit on mobile, use any app besides the official one. Personally, I use Bacon Reader.
If you're on desktop, use the old reddit layout, and install RES if you can.
Nearly every complaint I see about reddit's UX is because of people using the official app (Seriously, how is the official app the worst one?) or the new reddit UX.
As for Twitter, I've never felt its website or app to be slow on mobile.
If you access the website from a mobile browser it will nag you at every opportunity to switch to the app. It's quite annoying, so I wouldn't be surprised if most people give up.
You can use nitter instances instead of Twitter. UI on there is super lightweight and usable, something you hope the original one would be. Also you are not distracted by constant popups for logging in.