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I've submitted this to the Rampant github page a little over a week ago, and have played about 50 hours since I first reported this issue. Following is the mostly-original issue text.

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I started a new save when Factorio 0.15.x was released, and modified the settings to be a combination of the Rail World + Death World presets (I like a very aggressive biter presence). Map view:
Recently (around the release of ~0.15.20), we've been experiencing periodic slowdowns of the game. We played with Rampant on Factorio 0.14.x and experienced massive slowdowns later into the game but they were more persistent than the slowdowns we've been experiencing in 0.15.x. Every 3-5 minutes or so, FPS and UPS both will drop from 60 to lower 20s - 30s and this will persist for anywhere from 10 - 45 seconds, usually.

We've come to the conclusion that it's possibly related to biter attack waves. While the script update time for Rampant doesn't increase over 1ms during these slowdowns, the Entity update time usually reaches 40ms and has spiked to over 80ms before. The slowdowns appear to happen when the biters reach the engagement range of our turret defenses:
Given the density of biters on our save and the level of evolution, we almost always have at least one attack wave incoming, but when multiple waves hit at the same time the game starts to crawl.

I've uploaded the save + mods to Google Drive for testing, but I'm not certain that the issue can be resolved easily since it appears to be Factorio's entity handling code that is suffering under the stress.

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Since I submitted that, we've played about 50 hours more on the world and now experience persistent lag (~50 FPS/UPS normally), with dips down to ~15 - 25 FPS/UPS occasionally with the dips seeming related to the Biters. At this point I'm not too surprised that the game is lagging regularly because of how large the base is.
Here is the save in it's current state.

Thanks.

Does the FPS drop occur during the biter attacks, while the enemies are dying? I recently discovered the blood particle effects from killing enemies can lag the game. While it's at it's worst when they're visible on-screen, they're still pretty bad even when not visible.

I did report this, but it was rejected as 'not a bug' - viewtopic.php?f=23&t=45973

You could try the simple mod I wrote that disables these particle effects - https://mods.factorio.com/mods/jonatkins/AlienBloodFix

@jonatkins

Veden (author of Rampant) actually pointed that mod out to me on the Github issue, and it's in the second save. From what I noticed, it seemed that adding your mod helped reduce the duration of the slowdowns, but did not eliminate them.

Glad it's of some help.

I wasn't 100% sure if the particle FPS issue was just my PC, and I'd not had any feedback on the mod so far.

What I'm seeing is that a mod – presumably Rampant – creates some pretty huge unit groups and uses them to attack.



The mod's update time will be low since it only assigns commands to units, and the slowdown then comes from the biters' AI, which is accounted to “entity update”.

There is some room for optimisation here, but massive swarms will always tax the CPU. So I'll blame Rampant (or some other mod) for this.

Not a bug.