Weight and lap times

How much does the extra weight you gain through penalties affect lap times ?

I'm doing a STCC Championship and at the first event my lap times where the fastest by about a tenth or two. I managed to win both races with constant lap times and gained 50 Kg. Next race I'm not even top 20 with two seconds off. Got another 5 Kg overall and at the third event I'm again about two secs off. Tried multiple setups with no change.

I love the idea of the weight penalty, otherwise I wouldn't have choosen it, but do 50 Kg really add 2 secs ? Because fuel load doesn't seem to matter much as I did similar lap times with 40 or 5 liters.

Does anyone know the relation of weight/laptimes in the game ? It would help me to determine how far I'm really off.