Theres always android bloatware running in the background which is making the phone lag like Google Services, etc.
This kinda bloatware runs quietly and task killers cant really stop these tasks as they will re-emerge again after you stop it. That is why many people resort to custom roms to elliminate these problems but unfortunately custom roms also eliminate some of the required functions that makes android run.

Some custom roms do affect the phone in terms of functionality. For example, wifi not working, weak signal for network, Gps not working correctly for some phones. Of course the stock roms would be best for the phone but it would be loaded with bloatware.

todays technology is trying to make people healthier, health is the human future. Do anyone here still remember the Gorilla Corning Glass that is anti-bacteria?