Now I'm not 100% certain on the specifics but I do know that when large numbers of sprites and particles are spawned in a certain area of any online game it causes server strain. This is "not" mitigated by lowering your graphical settings because server side calculations have to take place for the particles and effects. You can see this a lot in the ettenmoors, 20 or 30 creeps and freeps will be lining up for battle but the lag doesn't start until everyone starts spamming skills (no matter how low you've dropped your graphics)... then it's lag city.
So my suggestion is as I stated up in the title. Can we have reduced particle effects in the ettenmoors? There's no reason to have flashy 1000 particle skills with a lifespan of 5 seconds when any creep with the minimum requirement of 1 brain cell can see that a skill has been used with only 250 particles and a lifespan of 3 seconds. Those are just random numbers pulled out of the air since I don't know the exact particle count, lifespan, and size each skill creates... and to be honest I'm not sure I even want to on some of these (especially the call of oreme)... but if the developers could reduce the particle count and lifespan it should greatly reduce the lag on the ettenmoors. Unless they're using an engine that renders particles differently from Unity, Doom Engine, Source, and Cryptic Engine.