that's because it jits code instead of pre-jitting like android does, so when you hit a piece of code that hasn't been compiled yet you perceive it as slowdown since the first time it runs it is emulated.
then it makes no sense why apple warns you that your device is performing "background" activities post update, last time I looked at iOS binaries they seem to decompile as-if it was bytecode - but it could also be related to more advanced debug directories haven't messed with internals of these devices since breaking the bootloader became pretty much impossible.
