TRB500: High speed in NAT-Mode but slower speed on passthrough-mode and bridge-mode

When i have my TRG500 in NAT-mode i have download speeds up to 800mbit/s and upload speeds up to 100mbit/s and that is all fine!

But I want to run bridge mode or passthrough in my setup.
In both of these modes I only get download speeds between 150-200mbit/s

I have read some forum posts that says try to enable/disable SYN flood protection, unfortunaly it makes no difference,
I have used tried with firmware TRB500_R_00.07.06.3 and TRB500_R_00.07.06.1, no difference.
I have tried to factory reset the TRB500, makes no diffrence
I have tried to connect a computer directly to the TRB500 (instead of via a rauter), makes no difference either.

Does any one else have any thought on what to try to be able to use all of my download speed?

Thanks in advance!