That depends if you use ads optimized for mobile phones or not, if not they will slow down the browser obviously, as they can be more demanding.
Sites overloaded with ads are a good example of an instant-slow-down due to required rendering via CPU, RAM or both.
True that, those site with and overload of f*cked up ads, I curse them

But talking about loading time and such.
When I currently access mybitmine.co directly from my phone, so am loading the website like I would load on my laptop/desktop, it takes about 2 sec MAX to fully load, and I almost instantly can spot the place where I can login

So the current state ain't that bad at all

(I do use 4G)