Check if Burst Length is set to 4 for Half-rate.

If you set Burst Length = 4 at Half-rate, the amount of data that can be transferred when issuing one command is halved, and the bandwidth drops by 50%.

When using DDR2 High Performance Controller II (DDR2 HOC II), make sure it is set to one of the following:

・Full-rate: burst length = 4
・Half-rate: burst length = 8

 https://www.altera.com/support/support-resources/knowledge-base/solutions/rd02142012_914.html