I have a 5K iMac. I believe the thunderbolt speed is related or shared over one bus. I have an external Lacie Thunderbolt drive, as well as a OWC ThunderBay 4, four-bay drive enclosure with dual Thunderbolt 2 ports. If I connect the Lacie into the Thunderbay 2nd port, I lose 75 MB/s write speed, but keep the same read speed. My second Thunderbolt port I use for an secondary monitor using a mini display port to DVI adaptor. My question is, how much data transfer does a monitor use compared to an external Thunderbolt drive? If i connect the monitor into the 2nd thunderbolt port, how much of a performance hit will I get?