Yet in my ROM, this lag is non existent, so it's not like it can't be done. So why doesn't Google address this out of the box?
With utmost respect, this is spoken like someone who has never worked in development
There are fixed resources. Only so many hours in the day, only so many people on staff, only so many dollars allocated to a project. So you must pick and choose what is important, what is not important, and what can feasibly be fixed.
While I am happy that your ROM has reduced lag in some areas, you honestly do not know at what cost. There are a lot of ways to change performance. Was code removed that slowed things down but prevented errors from occurring in other phones? Was the process given higher priority, meaning that other processes could be starved for CPU time if not enough is available? Perhaps the dev simply removed some optional code, which was important for Android as an ecosystem but not important for you or this particular ROM.
Changes are not as simple as you think they are. Frequently, changes affect other parts of the system, so resources and test cycles must be planned accordingly. I work in an IT shop that produces embedded devices, very similar to how cell phones are treated - this is how it is done. You prioritize, plan, and allocate resources to the best of your ability.
Sometimes that means seemingly "obvious" problems get shipped if they don't adversely affect core functionality because there weren't resources to correct and test (remember, everything must go through a comprehensive test cycle). Sometimes it means that a fix is slated for another release - so it's not that it wasn't identified, it's that the resources were planned for coming months. Sometimes you have problems that you simply can't easily fix - homebrew fixes don't always pan out on a broader playing field. Sometimes, yes, things just get missed. But more often than not, they're on a bug list somewhere that the project managers prioritized and decided to not fix yet.
