Some mods (types of car) push the boundaries of rFactor's suspension. It works fine for your car, and for everyone else's on their own computer, but rFactor's network code (which tells your game where the other cars are, if their headlights/brakelights are on, if their wheels are turning or skidding, and which way their car is pointing, plus other things) seems to make some simplifications that those suspensions 'break'. So you can end up with cars tilting to one side like that.
It's not a problem as such, and you can't fix it. It just looks strange.
You've said the program 'starts to lag', which can mean lots of different things (low framerates, skipping framerates, input lag, other cars lagging/warping, ...). If I were to take a blind guess I'd say either your PC is struggling to cope with other cars around (CPU can't keep up, and/or the GPU is struggling) or your network connection isn't great - making other cars move around unrealistically. That may also contribute to the cars tilting as shown.
A slow connection can do that, or an intermittent connection - either poor quality overall, or running wireless.
