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 channelâ€™s available 10Gbps bandwidth. Display traffic is one-way, so thatâ€™s why the read speeds from the Pegasus arenâ€™t as affected. To test this out, we attached the second Thunderbolt Display to the iMacâ€™s second Thunderbolt port and found performance to come back to the level of a setup with just one Thunderbolt Display attached.