The JTAG chain is as fast as the slowest device in the chain. Therefore, to lower the JTAG clock frequency, connect to a target device whose JTAG clock frequency is less than the default JTAG clock frequency.
You should attempt to open with a default JTAG clock frequency that is 15 MHz for the Digilent cable connection and 6 MHz for the USB cable connection. If it is not possible to connect at these speeds, Xilinx recommends that you lower the default JTAG clock frequency even further as described below.
To change the JTAG clock frequency, use the Open New Hardware Target wizard, from Vivado® Design Suite, as shown in the following figure.
Alternately, you can use the following sequence of Tcl commands:
connect_hw_server -url machinename:3121
current_hw_target [get_hw_targets */xilinx_tcf/Digilent/210203327962A]
set_property PARAM.FREQUENCY 250000 [get_hw_targets */xilinx_tcf/Digilent/210203327962A] open_hw_target