I downloaded and tested the provided mission, as well as tried some more details with the following result:

I can confirm FPS drops and stutters when driving past the rocks, regardless if there is one or more of them present in the mission. This issue does not seem to be influenced by "allowDamage" or "enableSimulation" editor flags/commands on the rocks itself.
However, the issue does not appear with the wavebreakers spawned as a simple object via createSimpleObject command.

This should probably be looked at by a developer, I think this might be an issue with the p3d-model itself.

Had this issue in Malden after adding those rocks manually via the EDEN Editor as regular objects (Not simple objects) but all simulations disabled on them.

My issue was kinda different thou, I would get one HUGE stutter when driving up to the rendering distance of the stones. After that one huge 2 seconds stutter (pause) the game would become normal and no further stutters would happen until the mission is restarted.

Whatever the case those objects surely cause performance issues.

I hope that if the culprit is found then the devs can check on other objects too that could be suffering from the same issue.

Important update:
This is a huge issue and could be responsible for lots of the perceived stutters and sudden slowdowns A3 is so famous for.

Basically the large rocks models, of which there are hundreds of in both Altis and Malden, cause significant stutters when ANY vehicle, including Ai units on foot, are near them.

I have updated a test mission to illustrate on the severity of the issue where I added just 22 of those rocks (Yes, just 22 will cause issues already) and then some Ai units on foot near them.

If the player shoots at those Ai units, even if the player is 800 meters away from the rocks and the Ai units, it will still cause stuttering as bad as 1 second each time the players causes damage to the Ai units (Yes, you heard right every shot that hits the unit will spark the stutter).

So if you hit a single unit several times you can have up to 2-3 seconds freezes! And it is even worst if you damage Ai units inside vehicles near the rocks.

Updated VR test mission with 100% replication of the issue:
https://drive.google.com/file/d/18GcHEzZwDZSlyxRFvaXM917m3PnOejO7/view?usp=sharing

The issue probably originates from the fact that those stones have higher polygon counts than they should, meaning that they are currently too complex and with too many polygons for the engine to handle well or that its collision calculations are too complex or both issues combined...

It is shocking that such gameplay altering issue has remain unattended for so long.

Even worst, the issue has even been exacerbated recently by custom compositions made available on the steam workshop a few patches ago.

Since the devs have not even warned players and/or content creators about those unnoptimized, stutter inducing objects on the Eden Editor many composition creators are using dozens of these flawed assets on their compositions and even mission makers are using them to simulating caves, underground facilities etc without knowing that by doing so they are basically killing the performance and the hurting the enjoyment of their playerblase tremendously.

If this won't be resolved then I believe at least a warning statement aimed at mission makers, composition creators and other Arma 3 content creators such as modders, map makers etc not to use those assets and/or remove them as selectable from the EDEN editor so they can’t cause further harm (of course all the maps that already use those objects would remain flawed), would be needed.

Interesting that you mention AI moving around impacting this issue.

I have run into similiar issues, but only in certain terrains and in slightly different circumstances. But all this reads like it could be caused by the same underlying issue.

In my case i observed that when i spawned AI at certain places fps were droping by around 50%, even so AI was not moving. But if i executed a doStop on these AI the fps came back. When i then reversed that doStop with a doFollow command the fps droped again.

AI moving around in other places of the terrain did not seem to influence performance at all.

Interestingly when using a dedicated server the fps droped only on the server, not on client side. Which looks to me that it might not be the polygon count itself but maybe some extra data that might also increase with a increased polygon count and may create that problem?

Will test the testmission posted by LSValmont later and see if i can reproduce it on my side too.

I have tried the test mission from LSValmont and can reproduce the mentioned issue.

However this testmission might not really demonstrate the cause of the issue properly, because beside the rocks and the AI it contains some other objects and especially the more unusual "hide terrain object" feature, and from them there are multiple ones.

If i remove the "hide terrain object" elements from the terrain the stutter is much less and might be even considered normal as it might cause by something else like loading something from harddisk.

I think the test mission should be stripped down to the essentials of AI + rocks to see if the problem is still there for you. Then you might check if just having the "hide terrain object" elements in the mission might cause the issue, even if the rocks are not present. Or it might be a combination of objects + "hide terrain object" feature that creates the stuttering.

Here is another video of the issue, completely destroying FPS and stuttering when shooting near these large rocks, this time in Gabreta.
https://youtu.be/4lgHVz2HGQM?t=272