Well, then. How about we go the way of the Solar Panels? Well, sort of. Make multiblock versions of the Geothermals. A 2x2x2 that puts out medium voltage and a 3x3x3 that generates high voltage, and each eat through lava at an appropriate rate. If people are just going to put out huge geothermal facilities and use transformers/storage devices to get the voltages and outputs they need, it would reduce server load a bundle.
Plus, it wouldn't be just a "no" to people who want that method of power, either for maintenance or thematic reasons. Coming from the tabletop RPG world, I can say that my players are always much more satisfied when I tell them, "Sure, you can do that. But..."
The lag comes from gathering the lava, which causes flowing liquid blocks, not using it in the generators :-D
Here's another fun 'fix' for the lag: change the lava-powered blocks into something you need to bring to the lava lake, instead of bringing the lava lake to it. I'm really not sure how feasible it would be to stop people from just making artificial lava lakes, but I think it would be nice to have more mechanics that require us to leave our base camps and set up outposts.