cd /.zfs/snapshot hangs (tmux put to uninterruptible sleep)

[Niklaas Baudet von Gersdorff]

When I cd into /.zfs/.snapshot the shell hangs. I must confess, there
are a lot of snapshots because I never thought this may cause a problem.
Maybe it does now.

$ zfs list -t snapshot | wc -l
    1316

These are hourly, daily, weekly backups. I keep them for some time but
delete old ones. They accumulate because I have several jails with a
independent datasets on the system. (In case someone wonders why there
are that much.)

Nonetheless, there are not that much snapshots on the tank/root dataset
that is mounted to /.

$ zfs list -t snapshot | grep tank/root | wc -l
     174

So a `cd /.zfs/snapshot` should only list 174.

Why am I not able to see the output of `cd /.zfs/snapshot`? Did I reach
the limit of possible snapshots?

Related to this problem: I ran the command in a tmux session that is now
freezed.

> $ ps -lJ 0 | grep 'tmux: server'
>  1001 21018     1   0  20   0 42396 19432 zfs      Ds    -       0:55.71 tmux: server (/tmp/tmux-1001/default) (
>  1001 86447 85718   0  20   0 18808  2236 piperd   S+   20       0:00.00 grep tmux: server

`kill -9 21018` doesn't kill the process. I cannot return to tmux with
`tmux a` either.

Any help is very much appreciated.

Best,

-- 
Niklaas