freebsd stable - May 2019 - mdconfig and the [no]async option

Hi.
I'm testing md devices with zfs and i see a slow performance on md0 and md1
(md1 by network).
Reading in man, i have the noasync option to up performance but the warning:

For vnode backed devices: avoid IO_SYNC for increased
    performance but at the risk of deadlocking the entire
    kernel.

So, can the operating system freeze if some write or read block fail ?.

Best Regards.