Shaders have to be processed when the video drivers are updated, and time to process will depend from game to game, how many shaders there are. After they are processed, shaders can be cached and recalled without a performance hit. But the cache will be invalidated after a driver update.

If you skip preprocessing, you may see a hitch the first time a shader is used in a game scene. Like if you pick up a gun that shoots blue flames, and the game hasn't used the blue flames before - it has to process that immediately before displaying the blue flames - which takes a split second.

This realtime impact can be small or large, depending how many shaders load into a scene simultaneously. Loading a new map with lots of unique textures and unprocessed shaders is generally when you'll see the big hitches as it scrambles to compile them.