There are some very poorly optimized rfactor 2 circuits. However, all the ones that rfactor2 has in his store, from Reiza, are perfectly optimized. Why can't they fix this?
Reiza tracks have a much older graphic technology. They lack many of the newer shader layers that all the S397 DLC content has. So it is not a matter of being poorly optimized, but rather the Reiza tracks just have less 'stuff' in them. However, recently Reiza announced they have begun updating their tracks to the latest S397 standards so whenever those get released, you will probably see the same strain on your system that the current S397 track demand.
There are poorly optimized circuits and there are circuits with a very high fidelity and detail. The DLC by S397 falls in the latter category. The Reiza circuits are old and haven't the level of detail, in the graphics and the terrain, that more recent official circuits have. The Reiza circuits aren't laser scanned while most modern DLC is. In laser scanned tracks the terrain is reproduced in full detail and that requires more resources. If you don't mind lowering the graphics detail level you can lower the level of detail of track and texture settings. Reiza is updating the graphics in its DLC, although the track won't be laser scanned still.