If it's really "by design" then you are saying they have a staff of web developers who are told, "No, no, no... all that quality work you're capable of--don't do it. Here are some JIRA tickets to make the web site shitty and slow and eat the user's battery. Go implement them and make everything worse!"

What kind of sad, self-loathing software developer sits down and says "OK boss, whatever you say, boss, gonna go make it bad now..." I mean, I know to a lot of people, it's just a 9-5 and you do what your boss says, and "pride in your work" is not really a thing anymore, but come on. Who gets even a shred of satisfaction doing this?

I think a better explanation is just incompetence.

It's usually done in such small portions the developers don't know exactly what they're doing. That, or they've become so numb to it to not really care