Union and Merging Slowing Performance Terribly

I am having the same issue.  The union function has slowed down my model dramatically.  I am joining multiple tables that each calculates supervisor hierarchies at different points in time (path function).  Have you explored simply a limitation with your machine?  I noticed that my CPU usage and Memory (20 Gb) is maxed out the entire time.  Prior to adding the union function, my model would max out resources but get through jobs in a manageable amount of time.   I was wondering if anyone on this string has used virtual machines, with more power, to refresh their data models - could that be a solution?  (I am a complete newb to virtual machines and looking forward to seeing what community has to say).  I am hoping there is a model solution and not a "throw more memory at it" solution.  Thank you!