This is a question I've been asked a few times. By default, SMS and ConfigMgr both compress package content into a PCK file to distribute the content to child sites. The problem you might run into with Operating System Deployment WIM files, which are already compressed, is that they take forever to move from one site to another, finally to your distribution point.
There is a way to handle the compression and exclude WIM files, as well as any other extension you want to exclude. This can save you a good amount of time. In my customer's case this week, we noticed that distribution manager took 5 minutes to complete "compression" instead of 30 minutes that it previously took.
I mention "compression" (in quotes) because while distmgr.log will show the file being compressed, if you look at the file size, it's actually slightly bigger (in some cases) than the original WIM file.
For example, look at this screen shot of my distmgr.log where I send an x86 boot WIM that distmgr compresses
Notice the first line where it says the size of the package is 129544 KBytes and the compressed size at the bottom is 129068 KBytes. Not very much space gained here, but look at the amount of time it took to compress roughly 130MB. It took 2 minutes.
Let's take a look at the same package with compression off for WIM files:
Notice here how the same package content is still "compressed", but the content is actually BIGGER, yet the amount of time is a minute less (or 50% in reduction of time to "compress"). It's bigger, likely because there is some additional information that gets added to the package as it is getting moved to a .pck file. The package will always convert from a .wim to .pck, however the compression engine simply isn't involved.
In order to exclude WIM files from compression you need to edit the following location in the registry:
HKEY_LOCAL_MACHINE\ SOFTWARE\ Microsoft\ SMS\ Compression\ DontCompressExts on 32 bit servers
HKEY_LOCAL_MACHINE\ SOFTWARE\ Wow6432Node\ Microsoft\ SMS\ Compression\ DontCompressExts on 64 bit servers
In this list, you will see: