Synchronized lag spikes affecting multiple players during races

Hi !
We have been experiencing performance issues during online races for the past few weeks.
Several players on the same server are affected, with major performance drops resulting in very severe stuttering.

The first time I experienced this was in a simple public lobby. Strangely, GPU usage would drop significantly, causing heavy slowdowns and FPS drops. This would happen intermittently but in a very regular pattern. On other races, everything would sometimes run perfectly fine.

When the issue occurs, it always happens during the race itself, while just before — in qualifying or free practice — everything was running perfectly smoothly, with exactly the same number of players on track.
This problem has also appeared on our private league server.

What is particularly strange is that the issue happens in a "synchronized" way for all affected players. Some players experience no lag at all, but for those who do, it starts at exactly the same time across everyone affected.

It does not seem related to a specific location on the track, but rather to specific moments (for reasons we cannot identify). For example: everything runs smoothly for about 40 seconds, then at the 41st second, the issue starts simultaneously for all affected players. We get a massive 2–3 second lag, then it becomes smooth again, stays fine for about a minute, and then the same synchronized lag spikes happen again.

During our second-to-last event, we raced on 3 tracks: only the first one caused issues, and the rest of the evening was fine.
During our last event, we were racing at Road America and ran 2 races. The problem occurred throughout the entire evening (for the affected players), but as usual, only once the race started. Practice and qualifying were perfectly fine.