Two Thunderbolt Displays Reduce Drive Performance

James Galbraith, Macworld:

The reason for the performance decrease in a two-display setup is two-fold. First, the Pegasus is fast and can put plenty of bandwidth to use. Second, each Thunderbolt Display gobbles up about 7Gbps of an outbound channel’s available 10Gbps bandwidth. Display traffic is one-way, so that’s why the read speeds from the Pegasus aren’t as affected. To test this out, we attached the second Thunderbolt Display to the iMac’s second Thunderbolt port and found performance to come back to the level of a setup with just one Thunderbolt Display attached.