While this article focuses on the ZDS Shifter, everything here applies equally to the Zendrum Stompblock.
A typical “pure” MIDI thru device can claim to have zero latency as the MIDI In connection is generally wired directly to the Thru/Out port. However, the Shifter (and the Stompblock) use a soft-thru.
Why a soft thru? It’s because these devices need to receive and analyze the entire MIDI packet (typically 3 bytes) before deciding what to do with it. In the case of the Shifter, this includes looking up and processing any “shift rules” associated with the message before retransmitting the message.
The Shifter is a low-latency device and the average time it takes to process an incoming message is 100 microseconds. That’s a tenth of the milliseconds in which latency is usually expressed and completely imperceptible.
However, that’s not the true latency of the device. Because as I pointed out, it needs to wait for the entire message before it can start processing it. This means the REAL latency is about 10 times that, but still only around 1 millisecond and generally below the level that a human can detect.
The conversion to USB adds on a bit more latency that varies in length as it needs to line up with the 32-bit (4-byte) frames that USB uses. On the high end, this can add another 500 to 800 microseconds meaning that the true latency of the Shifter ranges from 1 to 1.8 milliseconds. Your mileage may vary as you also need to take into account the hardware (and software if applicable) on the receiving end.
Generally speaking, if you’re connecting to a hardware sound module or a modern computer you should experience very low latency.