Please note: The forum, especially these bug reports, is not a place for personal disputes.
Regarding the 'lag'. The creature migration 'might' cause slower frame rate or command processing to back up, simply because more command are being processed. It is rare, however, for the creatures to have a significant effect.
The amount of objects in game (in particular building parts) will increase loading times. Intensive building will slow frame rate as a lot of objects are checked and rendered.
I've checked many busy areas in game and do not see any current issues. I am constantly trying to optimize functions to improve frame rate. It's difficult though in heavily built up areas.
If 'lag' has fluctuated, the cause must be connectivity issues outside of the game itself. If other players have experienced increased 'lag' in actions please let me know here. For now I will mark this as 'not a bug' as nothing has changed that would cause actual lag.
Thanks!