@enjay
There will always be a noticable difference between no-compression and any compression, the CPU is skipping a whole bunch of work -- but, there is no difference between something reasonably compressed and something compressed more tightly; this is a matter of what method the compressor used to examine the content. The same content can be compressed literally an infinite number of ways, so optimizers just try *many* more ways than usual to see if any come out smaller.
Using DOOM-Crusher
will not slow down resource loading on already compressed resources (such as PNG, PK3) -- and what's more PNG files are always compressed in some form, so DOOM-Crusher will always speed them up: the speed of the CPU is tens of thousands of times faster than the storage, hundreds of thousands of times faster than mechanical drives and USB.
I.e. if you want ultimate speed: use WAD & DOOM image formats, not PNG/JPG; but just because you can do that, that isn't a reason not to use DOOM-Crusher. DOOM-Crusher includes WADPTR which can 'de-duplicate' nodes and lumps in a WAD speeding these up too, without the use of compression.
The reason I made DOOM-Crusher is because I'm also making PortaDOOM and whilst shaving a few hundred K off of a resource might not be worthwhile, as soon as you get 100s of WADs in one place it starts making a notable difference. There's no way you can say that shrinking Blade of Agony by 69 MB is not going to make a difference

@wiw
It's a batch file that brings together a number of existing tools, see the page here for a description of these:
https://github.com/Kroc/DOOM-Crusher/bl ... /README.md