The serial nature of JTAG means that programming large flash devices can be very slow. If there is a programmable logic device (such as an FPGA) connected to the flash part, XJFlash can be used to dramatically speed up the programming process quite often reaching the maximum speed for the device.
The XJFlash module uses the internal circuitry of an onboard FPGA to create a simple flash programmer. This relieves the bandwidth restricted JTAG scan chain of repetitive tasks such as shifting in control, address and data bits. By removing traffic from the scan chain, the XJTAG system is able to achieve close to theoretical programming times for a given flash memory device.
If speed over JTAG is an issue, there are alternatives that can greatly improve programming speed, but since they are dependent on the specific application it is best to contact XJTAG to identify the best possible solution.