Hi msfujino
of course, thank you so much for chiming in.
The connection interval (not the advertising interval, but this problem only matters once connected) is actually 800 ticks = 1000ms, set by the (Arduino) central (but this device is already NCS/Zephyr, as written above):
int connInterval = 800;
connection_left->requestConnectionParameter(connInterval, connInterval);
updateConnParamsTicks(conn_hdl_left, connInterval, connInterval, 0, 600);