I have been putting together a fairly complicated piece and whenever I go to save it, it generates a render preview that takes about 15 minutes to complete before doing the save.  It is getting vey annoying waiting 15 minutes each time I do an intermediate save (which used to be pretty often, but not anymore).  I have searched thru the preferences but have not seen where this can be turned off.  How do I turn off the generation of this render preview?