Copy via thunderbolt

Hi all

I am using an iMac (Mid 2011) and a MBP (Mid 2014) both running Yosemite 10.10.3. I want to copy my Pictures folder from my MBP to the iMac. If I am using thunderbolt <-> thunderbolt (0.5m Apple cable), it takes days to copy 123GB. The iMac has a fusion drive (256GB / 2TB) and only 93GB are used.

Any ideas why it's so slow? If I copy it using Ethernet (with USB Ethernet Adapter) it's much faster.

Thanks,

Harem-Doctor