OK, so I've got a whole bunch of mods, but I think I've narrowed my lag down to water turrets. Whenever they attack the enemies, the whole game slows down and runs at about quarter speed until they have finished burning, and then it all speeds up again... until the next wave. If I remove water turrets, this problem goes away (but then I've lost that weapon)
Anyone else get this ?
(I'm using alien biomes, rampant, Krastorio amongst others)

Whenever they attack the enemies, the whole game slows down and runs at about quarter speed until they have finished burning, and then it all speeds up again... until the next wave.

You wrote about '"enemies" and "burning", so just for clarification: Does this also happen if biters/spitters are attacked, or only if turrets attack fire/acid? Did you notice the slowdown just with one kind of turrets or with all? There's a setting to let plain water turrets prioritize targets (prefer acid/fire, prefer enemies or attack whatever is in range automatically). Setting a priority means that the script will have to find suitable targets, so this definitely will be slower than auto-targeting.

It happens when I use steam to power the water turrets, and when the biters attack. They burst into flame, and the game lags while they're being shot at, but then returns to normal once they're done.
Hold fire (sorry) on this, and what I'll do is strip it back to just a couple of mods, and see if I get the same problem. I suspect it's a combination of water turrets, alien biomes and rampant, but I'll try and get more info to save you shooting in the dark

I have the same problem with Water Turret and Space Exploration. At the beginning of a game Space Exploration launches 3 CMEs which cause a lot of fire and it lags at this time. But I don't have this problem with Mod version 1.0.1 from Water Turret.....after updating to 1.0.3 the lags begin....

Yeah, looks to be Space Exploration, and maybe NE Enemies related. Same for me that it didn't happen before, but I'm currently testing mods against 1.1 and have found bugs in Space Exploration so can't complete the testing yet