Thank you for answer. I admit that I was a little carried away when I used my simple (misleading) test as a prerequisite for measuring performance. I underestimated a bit the number of operations that are associated with the measurement itself.
I did a new a little more accurate measurement through the ILA. I measured the difference between the S_AXI_ARVALID and S_AXI_RVALID signal on the AXI slave.
BRAM - 3 ticks (its nice, but this is not in my focus)
DDR2 - 22 ticks = 220ns
I'm probably wrong, but I'm based on the information in the Nexys A7 reference manual. Here is information about the transfer rate of 650Mbps at clk_period = 3077ps.
So my assumption was that read_frq = 650Mbps / 32bit => (650*1024*1024)/32 => 21.2992 MHz => aprox 47ns (read access time for one 32bit word).
Assumption according to the measured value of 220ns is 1 / 220ns => approx. 4.54 MHz => (1 / 220ns) * 32 => 138.7 Mbps