Comment 9 for bug 1760173

/*
 * Device creation by udev is asynchronous and waiting may be
 * required. Busy wait for 10ms and then fall back to polling every
 * 10ms for the allowed timeout (default 10s, max 10m). This is
 * done to optimize for the common case where the device is
 * immediately available and to avoid penalizing the possible
 * case where udev is slow or unable to create the device.
 */

That comment is not really correct though as udevd doesn't create
any device nodes itself in any recent versions.
The only thing it does is to create persistent names and symlinks.
The device node /dev/zfs is created by devtmpfs itself. Afaict,
udev only sets the permissions. Is the modprobe taking a long time?
Or is zfs somehow trying to go through a symlink that is generated by
udevd?