If you're in the market for a realistic racing game, Project CARS will not disappoint. Although, the performance issues exhibited on PS4 have prompted the developer, Slightly Mad Studios, to incessantly work on fixing those issues.
The head of Slightly Mad Studios, Ian Bell, has been very vocal on the official forums. As explained by Bell, they are pushing every core of PS4's CPU to greater than 99% under heavy load, and the GPU sits at greater than 95% at all times. In his posts, he also explains that the new 'tyre' model uses two full cores of the PS4. This model is a “full dynamic tire simulation,” as stated on the World of Mass Development website. It simulates realistic changes in tires, which effects driving in multitude of ways. This includes heat transfer simulation, which handles heat flow between brakes, wheel wells, rims, etc. As a result of heat transfer, changes in tire behavior due to surface differences is possible. This is an example of how resource intensive a simulation of true tire physics can be. Again, this is also one reason why Project Cars is such a resource hog.
In an article from Eurogamer, a performance test was setup between the Xbox One and PS4. The PS4, as well as the Xbox One, has trouble handling weather effects. This problem gets worse as the rain increases in intensity. The heaviest rain effects can bring the frame rate close to, or below, 30 frames per second.
Project Cars is robust and realistic racing game that holds nothing back. Seeing as they have an entire model that controls tire physics, I can understand why it would be intensive. However, I'm sure Slightly Mad Studios will be up to the task of fixing the performance issues.
Source: Forum Post 1, Forum Post 2, Tyre Model Page, and Eurogamer