Warn, if zappending will be slow

[forman]

Is your feature request related to a problem? Please describe.

Some configurations may result in very low slice appending speed, e.g., due to

• heavy rechunking involved, or
• slow remote slice or target filesystems.

Describe the solution you'd like

Emit a log message, warning level, if zappending will presumably be slow.
Likely this can be determined after the first slice dataset has been seen.
If possible, include hint to mitigate, or refer to user guide.
Therefore, mention common situations in the new performance guide, see #66.