With a little thought, making a Cursed Earth farm that doesn't run constantly and/or kill your framerate is easy enough. Some Redstone Lamps (or any other variant) can provide togglable light to stop spawning when it is not needed. Within a pack such as Infinity, this can be easily be automated (BuildCraft gates come to mind) to only allow spawning in your farm when the level of Mob Essence in a tank is below a desired threshold.
Really, your farm for anything below full scale power generation can be quite minimal in footprint. I only mentioned my power gen system to prove that it is possible to make a self sustaining system (or more, if you have the will). That said, the server we had the Mob Essence power system set up on was not particularly strong (run piggybacking off the computer of my friend, who played regularly), and we didn't have a huge impact on playability until we had a good deal of tech in the area as well as the farm itself.
I'll not dismiss the fact that entity count and AI do contribute highly to performance loss, but a simple 5x5 Cursed Earth dark room with a MFR Grinder or equivalent is not nearly as hard on a server as that same amount of space filled with machines ticking away. As with all things, moderation is the key.