I hate the 1 unit thing most of the time, to be honest. Yes, it can help, but you have to be really careful when and where you use it. Some people over use it and end up making things worse (not to mention look bad). It also makes the map files really messy. There are two key things that impact rendering speed: polygon count (r_speeds) and fill area. If you hover a large box over the ground or make it a func_wall to reduce polygon breakage, you're introducing something called overdraw. The game draws the ground, then draws the box on top of it, so it has to fill those pixels in twice. The BSP structure is designed to minimize overdraw (which is what slows a lot of low-end cards down), so it will split the ground up around the box in order to avoid drawing the same area twice.
The one unit gap thing can be useful for small details, though. I used it on the lights in the tunnels on midnight2, for example. The way they're positioned, it's impossible to tell there's a gap, and since they're small, overdraw is pretty negligible.
Back to some things to reduce r_speeds: use water sparingly, if at all. Avoid water at different levels, and never put it at angles or vertically. Even when water is not reflective, it has to be subdivided a bunch in order to create the swirly effect (previously this wasn't accounted for in the r_speeds, but I made the new version count these polygons). Transparent water also increases overdraw a lot, since the area under the water has to be drawn, then the water on top of it, so it slows things down in two ways (plus all the reflections).
