The only other thing would be internet speeds or how many others in the household are using internet connected devices at the same time you're logged on too.
What it was is it seems when I do laps in my lounge my times are slower than in my mates lounge with the exact same settings and just us 2 in them.
My friend has noticed aswell
you have different internet.. and GT5 have some sort of problems with this.. lap times are different in each lobby it seems.. not 100% sure.. I dont see it as a biggest GT5 online problem. Open lobby Freeze is much worse..