Light entities that have targetnames require VRAD to compile a second lightmap "page" for all surfaces that the emitted light touches. This will typically not harm runtime performance, but will increase compile time and BSP filesize. Also, in and effort to reduce lightmap pages, VRAD will not bounce light off surfaces for named lights, which can look very crude. If too many lights, or sets of named lights are present in an area, VRAD can become overloaded. This usually happens in the latter, where the number of pages needed increases exponentially as each set of named lights increases. For best practice, lights that will switch on and off together should have the same name.