I posted same question on Microsoft community, but no M.S representative have replied me over there yet.
Any ways, regarding the lag, Its general over all slow. Specially when you switch between apps even when there's not much going on in the back ground. While uploading videos on Whatsapp, it gets slow too.
When I open Onenote to note down something there's this annoying lag.( The logo of Onenote appears for a while) It should be instantaneous j like sticky notes in Android devices.
Also the health app is very slow.
Furthermore Multi tasking is not so mature. It sometimes doesn't resume from where i left the app. Back ground download & upload is almost impossible. So what's the use of having 3 GB Ram on your phone?