Depends on the complexity of the page. A lot of sites with heavier JS and SPAs eat a lot of memory which can cause problems for users of the many laptops out there with 4/8GB of RAM, as well as many smartphone users who have 2GB of RAM or less. In the case of the latter visiting a heavy website can be enough to prompt the OS to kill some other app to make memory available, which means in that situation one's site is in direct competition with other things the user may be needing more than the site.
> Does it though? Loading a webpage barely registers in cpu usage etc on a reasonably modern device.
CPU usage is not the problem. In most cases the problem is latency including the network latency to issue and return a substantial number of remote API requests across the Internet to get the data necessary to render the page.
In many cases unless you make page specific APIs that aggregate all the necessary data into a composite object on the server side this is the number one thing that slows things down. Network turnarounds are expensive but they are a lot less expensive when made inside of a datacenter than from a thousand or more miles away.
Aggregating the data requests is pretty easy if you use something like GraphQL + one of the clients (Apollo or Relay). Probably other frameworks can do it too, I'm too lazy to check.
Does it though? Loading a webpage barely registers in cpu usage etc on a reasonably modern device