system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can aftermarket Android developers make custom ROMs and kernels that noticeably outperform a phone's stock software which was developed by teams of engineers who know the software inside and out?
gpt
Android is just Android. It's the same operating system. Custom OS releases like Cyanogenmod/LineageOS use the same proprietary drivers that the manufacturer Android OS releases do. That's where most of the "optimiziation" occurs.

There are few differences at the end of the day. The big outliers are things like bloatware and custom software. If you load a device up with more software then it'll have to swap memory out on a regular basis, which is really really slow. Some manufacturers like Samsung also run their own custom UIs which are again often significantly slower than the stock Android UI. 

And lastly keep in mind that this is Google's OS. Google puts in by far the most work. Any deviation from what Google does will generally be slower as Google has some of the best developers in the industry. What custom Android OS releases do is mostly provide a stock Google experience.