But it affects user experience.

Backend performance issues can grind your system to a halt. It’s basically a requirement for Discord to work reliably.

Front-end performance is not a hard requirement for most end users, unless the app is actually unusable. Discord isn’t that bad compared to some software I’ve used. You have to get beach balls on startup and complete UI freezes for people to really care. If it’s good enough for most people, shaving some MB off the memory usage or small number of ms off latency isn’t important to the business

As long as negative experience doesn’t meaningfully impact user monetization, there’s no business incentive for that type of company to care.