tl; dr - The zig build run version appeared to run faster because it wasn’t waiting on echo or xxd to feed input to stdin.
To close the loop on this, Andrew Ayer explained the phenomenon I was observing.
My mental model was that echo ran, then piped its results to xxd, then xxd finished and piped its results to my Zig program.
But actually in a bash pipeline, all the commands in the pipeline start at the same time.
The reason why I measured slower performance in the ./count-bytes invocation was that it was actually running faster. The binary started the timer immediately and then had to block on the countBytes call for input to arrive from xxd, which hadn’t started producing output yet.
In the zig build run version, what was probably happening was that calling zig build run rather than ./count-bytes meant that count-bytes timer started later, as the overhead of zig build probably adds 100+ms. So, instead of count-bytes starting immediately and then having to wait ~100ms for echo and xxd to get input into stdin, by the time the zig build run version starts the program, echo and xxd have finished their work, so count-bytes doesn’t have to wait to read from stdin.