Because this problem involved Emacs, so I Cc Emacs mailing list too.
The problem is the commit "e1f09607e0" auto convert PDF file link in Org Mode when toggle inline image display. This used 100% of CPU by "gs" command.
I finally figured out the reason.
I'm using the latest "master" branch source code compiled Emacs. And it add new feature to convert possible image files.
The commit "e1f09607e0" caused this problem. I confirmed by git checkout a commit before it. And re-eval source code, then the problem is gone. This is because the commit added ~image-converter-file-name-extensions~ in image-convert.el library which includes "pdf" file extension. It uses command "gs" to convert pdf to image which use 100% of CPU.
I thinked about this problem, maybe this should modify code in Org Mode side to exclude new ~image-converter-file-name-extensions~.
Or does anyone have better solution?
[stardiviner] <Hack this world!> GPG key ID: 47C32433
IRC(freeenode): stardiviner Twitter: @numbchild
Key fingerprint = 9BAA 92BC CDDD B9EF 3B36 CB99 B8C4 B8E5 47C3 2433