Back in the days OS's ran off of floppies (Amiga OS for example) there were programs that would compress an executable to save space. This made me think of that using encryption instead. Now in that case on 7Mhz machines. The speed of decompression in that case was close to the time saved by loading less data from the floppy drive.
Now with Ghz of speed instead of Mhz and it needing to be run once, I am not convinced the speed loss would be that great.
However with VM's and the like it's too easy to stop and look at the executable that way - nothing can stop that.