Just as a PSA, the feature is currently somewhat bugged and really should be avoided. For anything that's not a low-end PC, your machine can handle the compilation during runtime easily and do it much faster.
For low-ends, it compiles so many unnecessary shaders (such as all workshop content that you might not even have), it often takes 10x longer to compile everything (which you have to recompile on every driver or game cache update) than just playing the game and watching a replay first or something.
The problem with fosselize is that it's currently bugged, and happens to precompile way more things than are needed, such as all workshop content that you might not have installed which takes a really long time + bloats up the shader cache in size. On anything that's not low-end, it's pretty much a waste of time since shader compilation is easily done on runtime.
Some issues on the things I've mentioned that Valve hasn't seem to have responded yet: bloat, time