I think they said it was something to do with maximising the power from used batteries.

Newer versions of Android will slow down your phone/ tablet, of course they will, a program designed for a octa-core phone will run slower on a quad-core and slower still on a dual-core.

The only phone OS that was very efficient and ran smoothly on all their devices was Windows Phone 7 & but that never caught on with the public, by the time they got to Windows Phone 10 it was laggier than Android.