CreateProcess is only being called once in this case i.e. to spawn svnadmin. The exact script does the following:
svnadmin dump d:\repo > repo.dump
The output from svnadmin has a lot of lines. Due to the fact that PS is written on top of the CLR, it reads each line into an immutable string before writing it to a file. So for every line it has to create a new System.String object and as another poster said GC it later. Also as lines are not predictable length it has to buffer them resulting in more overhead.
Effectively where *NIX shells use a fixed size buffer for pipe operations and operate on streams, PS has to convert it to lines first before writing it out.
That doesn't work when you have approximately 25 bytes per line and a 12Gb file which is where the issue is.
