The USB-Blaster II download cable supports TCK frequencies up to maximum of 24MHz. However, achievable performance is dependent upon board trace delay between the JTAG header and target FPGA device and device specifications. If you encounter JTAG configuration failure on your board, reduce the TCK frequency to 16MHz or 6MHz. Please refer to USB Blaster II Download Cable User Guide for timing constraint detail.
You can modify the USB-Blaster II JTAG frequency with the following command:
> jtagconfig --setparam <cable_number> JtagClock <frequency>
where <frequency> is either 6M or 16M.