why is my new MacBook Pro 2015 having such a hard time editing multicam proxy files?

FCPX has been behaving erratically and bogging down painfully. I've been editing a few different projects, some are multicam 4k interviews (SONY xavc-i) using proxy files and another project using gopro and 5D footage proxy files that is processing nearly just as slow. Im editing off external drives (Gdrives and Lacie) USB 3.0 and thunderbolt. I did an export benchmark test and there were no issues there. Working within the project is where I have most of my issues. Dragging on simple effects such as a cross dissolve or color correction can take up to 10 seconds to appear on the clips. Beach balls for days. using proxy files buys me a bit of speed but after a few hours of editing with those, I encounter these same issues. the strangest part of this all is these issues are not consistent. sometimes they may take a few minutes or hours to manifest. can you see an obvious bottleneck in my setup / workflow or do i need to get office space on my setup and take it to a field with a sledge hammer?

My specs:

Macbook Pro mid 2015 2.8ghz i7

RAM: 16GB 1600mhz DDR3

GRAPHICS: Intel Iris Pro 1536mb