I've never understood why client-side execution is so heavy in modern web pages. Theoretically, the costs to execute it are marginal, but in practice, if I'm browsing a web page from a battery-powered device, all that compute power draining the battery not only affects how long I can use the device between charges, but is also adding wear to the battery, so I'll have to replace it sooner. Also, a lot of web pages are downright slow, because my phone can only perform 10s of billions of operations per second, which isn't enough to responsively arrange text and images (which are composited by dedicated hardware acceleration) through all of the client-side bloat on many modern web pages. If there was that much bloat on the server side, the web server would run out of resources with even moderate usage.

There's also a lot of client-side authentication, even with financial transactions, e.g. with iOS and Android locally verifying a users password, or worse yet a PIN or biographic information, then sending approval to the server. Granted, authentication of any kind is optional for credit card transactions in the US, so all the rest is security theater, but if it did matter, it would be the worst way to do it.