Why do we get errors when we try to serialize a Hashtable?

XmlSerializer will refuse to serialize instances of any class that implements IDictionary, e.g. Hashtable. SoapFormatter and BinaryFormatter do not have this restriction.

Why is XmlSerializer so slow?

There is a once-per-process-per-type overhead with XmlSerializer. So the first time you serialize or deserialize an object of a given type in an application, there is a significant delay.

This normally doesn't matter, but it may mean, for example, that XmlSerializer is a poor choice for loading configuration settings during startup of a GUI application