When building shaders Unity enumerates through all possible shader variants to be able to strip the excess variants using scriptable stripping. With the amount of multi_compile features increasing in the shaders, the full variant space can grow into sizes where the time and memory requirements to process becomes unbearable.

URP is especially hurt by this due to how much it utilizes multi_compile. Take the URP template project and build it. A significant portion of the time goes into preparing 400M variants, after which we only compile the required ones. Now add a few multi_compiles on top of that and the preparation can take hours.