Gun out, when not done for a while, ADS'ing while have an unusually long delay while on a server.
This delay gets increased further by whatever your ping to the server is.
To be precise, the action of ADS'ing is delayed but the animation is not played, instead it will be instant.
This will get you killed easily in a crucial situation (i.e. suddenly running into somebody and it takes you over 1 second to raise your gun)
Example: (in this clip it takes over 1 second before my character even does anything, but at the end my gun is instantly raised and I can get a shot off)