What is the fastest OpenOCD compatible JTAG interface available on the market?
I don’t think there is any comparison for this type of thing due to the costs factor. You can try the IDAP-Link http://embeddedsoftdev.blogspot.ca/p/idap-link.html. it’s fast enough. It’s a CMSIS-DAP compatible JTAG that supports both SWD & JTAG mode. Works with all Cortex-M series supported by OpenOCD or any other IDE (Keil, CorssWorks,…).