I think people are forgetting that despite the pretty graphics that ESF now has, it's still an HL1 mod. I know it's still pretty different compared to a traditional HL1 mod when it comes down to performance, but I feel like most graphics cards should still be absolutely fine with the game.
Meeh..Sky don't scare me like that^^ thought something might went wrong
Oh btw Particles being heavy to render is not related to HL
Is a common problem artists have to tackle. Some bigger companies released articles on how to trim the particle geometry to squeeze out every bit of performance they can. Like the cloud particles in just Cause 2 or particle FX in Prototype.
In general transparent stuff is not that nice for gpus