You're right that the RAM keeps stuff in memory that you are using, i.e. background tasks while switching between programs, but if it gets full and it has to close apps and re-open the lag is from the app re-opening from the storage memory which is much slower than RAM. So that time delay is the app re-caching to the RAM from storage, not necessarily the CPU/GPU.
Yeah I agree,but I was thinking more in terms of general smoothness (animations etc., which people think of when they say their iPhone runs perfectly smooth), and that isn't really effected by RAM.