I have 4 hours of 12 tracks at 48 and every time I open it to break another song into a child project my drive spins to death.
 
I've been doing this same kind of project long enough to notice that things seem to have gotten worse recently.
 
I'm guessing all the Sonar developers are using SSDs these days and perhaps they've started to make choices that presume I am too.
 
Time to upgrade?

A file that size is most likely fragmented pretty badly. Iobit Smart Defrag is a nice free utility which will allow disk optimization. It can be slow, so best to do when stepping away from the computer for a bit.

I would recommend using something simpler, like Audacity or Wavosaur to break up the WAV file first. Sonar may be trying to do things that a simpler wave editor won't.
 
Regards,
Dan