how low-latency is the ? with USB involved, you'd think the answer is "not very".

not so much! on a workload involving interactive JTAG probing of a target (i.e. it's not synthetic, between the received packet and the transmitted packet the FPGA actually talks to a DUT) the lowest latency I'm seeing is ~80µs, which implies you get the answer in the same microframe you send the request

(it takes about 40 µs to actually perform the work, in this scenario)

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